President Barack Obama on Wednesday launched a staunch defence of the nuclear deal the United States and five other world powers reached with Iran.

Speaking at a press conference at the White House, Mr Obama sought to push back on every single argument critics had lobbed at the deal, at one point even asking and answering his own question.

He stressed that his number one priority had simply been to ensure that Iran did not get a nuclear weapon and that this deal was a historic opportunity to achieve that goal.

"This deal is our best means of assuring that Iran doesn't get a nuclear weapon. And from the start that has been my number one priority, our number one priority. We got a historic chance to pursue a safer and more secure world, an opportunity that may not come again in our lifetimes. As president and commander in chief, I am determined to seize that opportunity," he said.

The deal - which grants Iran sanctions relief in exchange for limits on its nuclear programme - had come in for fierce criticism from Republicans since it was announced on Tuesday.

Here is a summary of the main criticisms against the nuclear deal so far and how Mr Obama responded to them:

1.The nuclear deal provides Iran with a large cash windfall which it will use to fund its destabilising activities.

Mr Obama: "It is a mistake to characterise our belief as thinking that they will just spend it on daycare centres and roads and paying down debt. We think they have to do some of that because (Iran President Hassan) Rouhani was elected on the premise of improving the economic situation inside of Iran. That economy has tanked since we imposed sanctions. The notion that they are going to immediately turn over US$100 billion (S$136 billion) to the IRGC (Army of the Guardians of the Islamic Revolution) or the Quds force, I think runs contrary to all the intelligence we've seen.

Do we think that with the sanctions coming down, Iran will have some additional resources for its military and for some of the activities in the region that are a threat to us and to our allies? I think that is a likelihood that they've got some additional resources. Do I think it's a game-changer for them? No.

They are currently supporting Hezbollah, and there is a ceiling, a pace at which they could support Hezbollah even more. Can they potentially get more resources there? Yes. Should we put more resources into blocking them from getting that assistance to Hezbollah? Yes. Is the incremental additional money that they've got to try to destabilise the region, is that more important than preventing Iran from getting a nuclear weapon? No."

2. The deal will not make Iran more moderate.

"This deal is not contingent on Iran changing its behaviour, it's not contingent on Iran suddenly behaving like a liberal democracy. It solves one particular problem, which is making sure they don't have a bomb.

And the point I've repeatedly made and which, I believe is hard to dispute, is that it'll be a lot easier for us to check Iran's nefarious activities, to push back against the other areas where they are against our interest or allies' interests, if they don't have a bomb…

We'll be engaging with them, although keep in mind that unlike the Cuba situation, we are not normalising diplomatic relations, so the context will continue to be limited. Will we try to encourage them to take a more constructive path? Of course, but we're not betting on it…

But the argument I've been already hearing, and this was foreshadowed even before the deal was announced, that because this deal doesn't solve all those other problems that that's a reason for rejecting this deal. It defies logic, it makes no sense and it loses sight of what was our original number one priority."

3. Syrian leader Bashar al-Assad and leaders in Teheran are praising the deal, but American lawmakers and allies are criticising the deal. This is a sign that the deal is good for Iran but bad for the US.

"It does not give me pause that Mr Assad or others in Teheran may be trying to spin the deal in a way that they think is favourable to what their constituencies want to hear. That's what politicians do and that's been the case throughout…

With respect to Congress, my hope - I won't prejudge this - my hope is that everyone in Congress also evaluates this agreement based on the facts, not on politics, not on posturing, not on the fact that this is a deal I bring to Congress as opposed to a Republican president, not based on lobbying, but based on what's in the national interest of the US. And I think if Congress does that, then in fact, based on the facts, the majority of Congress should approve of this deal.

But we live in Washington and politics do intrude. I am not betting on the Republican Party rallying behind this agreement. I do expect the debate to be based on fact, not speculation and misinformation…

For all the objections of Prime Minister (Benjamin) Netanyahu, or some of the Republican leadership that has already spoken, none of them have presented to me or the American people a better alternative…

There really are only two alternatives here. Either the issue of Iran obtaining a nuclear weapon is resolved diplomatically, through negotiation or it is resolved through force, through war. Those are the options."

4. We could have negotiated a better deal.

"OK, what does that mean? The suggestion among a lot of the critics has been that a better deal, an acceptable deal, would be one in which Iran has no nuclear capacity at all, peaceful or otherwise.

The problem with that position is that there is nobody who think that Iran would or could ever accept that and the international community does not take the view that Iran can't have a peaceful nuclear programme. They agree with us that Iran can't have a nuclear weapon.

We don't have diplomatic leverage to eliminate every vestige of a peaceful nuclear programme in Iran, what we do have the leverage to do is to make sure they don't have a weapon. That's exactly what we've done."

5. The agreement doesn't do enough to stop Iran from developing a covert nuclear programme. The agreement says inspectors have to give 24 days notice before inspecting a suspicious site.

"Keep in mind, we will have 24/7 inspection of declared nuclear facilities. That entire infrastructure that we know about, we will have sophisticated 24/7 monitoring of those facilities.

What if they try and develop a covert programme? One of the advantages of having inspections across the entire production chain is that it makes it very difficult to setup a covert programme. There are only so many uranium mines in Iran and if we're counting the amount of uranium being mined and suddenly some is missing on the back end, they've got some explaining to do…

But let's say Iran is so determined that it wants to operate covertly. The IAEA will have the ability to say, 'That undeclared site, we're concerned about it. We see something suspicious', and they will be able to go to Iran and say they want to inspect it. If Iran objects, we can override it and we don't need Russia or China in order for us to get that override. And if they continue to object, we are in position to snap back sanctions and declare Iran is in violation and cheating.

As to the fact that it may take 24 days to finally get access to the site, the nature of nuclear programmes and facilities is such that this is not something you hide in a closet. It's not something you put on a dolly and wheel off somewhere… By the way, if there was nuclear material on that site, high school physics will remind us that that leaves us a trace. And so we'll know that in fact, there was a violation of the agreement.

This is the most vigorous inspection and verification regime by far that has ever been negotiated… The only argument you can make against the verification and inspection mechanism that we've put forward is that Iran is so intent on obtaining a nuclear weapon that no inspection region and verification mechanism would be sufficient because they'd find some way to get around it because they are untrustworthy. If that's your view, then we go back to the choice we had to make earlier. That means you can't negotiate. What you're really saying is that you have to apply military force to guarantee that they don't have a nuclear programme."

6.The US should not have agreed to the ending of the arms and ballistic missile embargo, even though it comes with a five- and eight-year delay.

"Under the terms of the original UN resolution, once an agreement was arrived at that gave the international community assurance that Iran didn't have a nuclear weapon, you could argue just looking at the text that those arms and ballistic missiles prohibitions should immediately go away.

But what is aid to our negotiators was, given that Iran has breached and the uncertainty of our allies in the region of Iran's activity, let's press for a longer extension of the arms embargo and the ballistic missile prohibitions, and we got that.

Part of the reason why we were willing to extend it only for five, let's say, instead of a longer period of time is because we have other UN resolutions that prohibit arm sales by Iran to organisations like Hezbollah. We have other UN resolutions and multi-lateral agreements that give us authority to interdict arms shipments from Iran throughout the region."

7. The deal ignores the four Americans still detained in Iran.

"If the question is why we did not tie the negotiations to the release, think about the logic that creates. Suddenly Iran realises, you know what, maybe we can get additional concessions out of the Americans by holding these individuals.

But we are working every single day to try and get them out and won't stop till they are out."

8. Even if Iran doesn't cheat, it will be able to develop nuclear weapons when the provisions of the agreement end in 10 to 15 years.

"There is no scenario in which a US president is not in a stronger position 12,13, 15 years from now if in fact Iran decided at that point they still wanted to get a nuclear weapon.

Keep in mind we will have maintained a one-year breakout time, we will have rolled back their programme, frozen their facilities, kept them under severe restrictions, had observers, they would have made international commitments supported by countries around the world and if at that point, they finally decided openly they are pursuing a nuclear weapon, they are still in violation of this deal and the commitments they've made internationally. And so we are still in a position to mobilise the international community to say 'No, you can't have a nuclear weapon'.

And by the way, we haven't given away any of our military capabilities. We're not in a weaker position to respond."