WASHINGTON - Tighter sanctions aimed at reining in North Korea's nuclear weapons and missile programmes are unlikely to work, given that Pyongyang has repeatedly said its ambitions are non-negotiable, analysts say.

But sanctions are the only option available to the international community to put pressure on the regime of North Korean leader Kim Jong Un.

Most recently, North Korean Foreign Minister Ri Yong Ho reiterated at the Asean Regional Forum in Manila on Monday (Aug 7) that his country "will, under no circumstances, put the nukes and ballistic rockets on the negotiating table".

"North Korea has said it won't give up nukes, they have made it explicit and very clear. It's time to believe that," Dr Sue Mi Terry, a North Korea specialist with the consultancy Bower Group Asia, told The Straits Times.

On Tuesday (Aug 8) morning, US President Donald Trump tweeted: "After many years of failure, countries are coming together to finally address the dangers posed by North Korea. We must be tough & decisive!"

Hours later as a Washington Post report emerged saying US intelligence agencies believe North Korea has developed the capacity to miniaturise a nuclear warhead to fit inside an intercontinental ballistic missile (ICBM), the US President ratcheted up the rhetoric, saying any further threats from Pyongyang will be "met with fire and fury and, frankly, power the likes of which the world has never seen before."

In fact the US is "cornered", Dr Terry said.

Dr Terry, a former CIA analyst, met representatives from North Korea in Sweden in June along with a group that included delegates from Japan, China and South Korea in the so-called "Track 1.5" diplomatic initiative. She wrote afterwards, "We left more pessimistic than when we arrived."

In the absence of other options, the US had to respond, she said this week in a phone interview. However, she added that there should be no illusions about the outcome.

"Tightened sanctions make us feel better and it's a last-ditch effort, but I'm not even sure people pursuing it actually think its going to lead to denuclearisation," she said. "If (the US) wants talks, we have to drop the insistence that they (North Korea) denuclearise. But the US is not going to do it, that's why we have this impasse."

"North Korea will continue on its path, it will conduct more ICBM tests and probably even a nuclear test, until they feel they have completed their programme and are capable of attacking mainland USA with a nuclear weapon," she said. "Unfortunately, we're stuck and this just has to play out."

Theoretically, the US could drop its preconditions for dialogue and enter into talks with North Korea. But that is highly unrealistic, analysts say.

For one thing, it is not certain that North Korea wants just a peace treaty. Mr Kim's goal could be to use nuclear blackmail to force the US to withdraw troops from South Korea, in effect abandoning Seoul, an outcome that is "unlikely but not inconceivable", said Dr Sung Yoon Lee, Professor of Korean Studies at the Fletcher School of Law and Diplomacy at Tufts University in Boston.

The sanctions - if fully implemented - will hurt North Korea to the tune of US$1 billion (S$1.36 billion) in lost revenue, but will not change Pyongyang's calculus. There are also questions whether the sanctions will be full implemented.

"In the short term, they will have no real impact," Dr Lee said. "The assumption that all states will faithfully implement them is unrealistic. They are difficult to enforce. North Korea has no reason to be deterred. They will probably go ahead and conduct another ICBM or even a nuclear test."

Mr Anthony Ruggiero, a sanctions expert and Senior Fellow at the Foundation for Defence of Democracies in Washington, suggested that robust sanctions had not been tried against the North Korean regime, noting that the actions taken against Pyongyang are far less even now than those applied to force Iran to stop its nuclear weapons programme.

Mr Danny Russel, Senior Fellow at the Asia Society Policy Institute, told The Straits Times on Tuesday:"There is some risk of a stampede, where people who are seeing - or guessing at - significant progress by the DPRK, are prepared to abandon a basic strategy of denial, pressure and containment. The question then becomes, 'What's the alternative?'"

"Up until now, serious full-on sanctions have never been tried. They've been a series of half measures. So the notion that sanctions don't work is misleading."

"There's no question that sanctions alone are inadequate to compel North Korea to shift gears and to negotiate an end to its nuclear programme."

"But there's also no question that creating maximum obstacles and headwinds to impede North Korea's ability to obtain the currency and technology it needs to advance the programme, and also to put the DPRK system under maximum stress, is an important component of slowing if not stopping Kim Jong Un and creating conditions that will cause him to have to make some tactical accommodations."

"This is a war of attrition. If the last thing on Earth Kim Jong Un wants to do is abandon his nuclear missile programme then it's incumbent on us to make it the last thing on Earth available to him to stave off the collapse of his regime."

Ms Yun Sun, a Fellow at the Stimson Centre in Washington, told The Straits Times: "We are looking at a strategic tightening of North Korea's revenue from foreign trade. If the purpose is to show North Korea your provocation has consequences and you will be punished, it will have that effect."

"But are we looking at North Korea really giving up nukes? I don't think so. Slow down their nuclear programme? Maybe, but they already have ICBMs and nuclear warheads."

North Korea is "almost invincible because they are no longer vulnerable", she said.

Yet when North Korea feels it has perfected its nuclear missile capability, it may be ready for talks. It would then be up to the US to agree.

"I'm afraid this crisis is just beginning. It's going to get more serious. I don't think North Korea is stoppable at this point," Dr Terry said. "Their calculus is hurry up and get to the point of perfecting their nuclear arsenal and then they will talk; at this point when they are so close, they won't stop."