Singapore's Prime Minister Lee Hsien Loong has voiced concerns about the current American political climate in an interview with the Wall Street Journal editorial board this week, as he continued to stress that the United States leadership remains indispensable in Asia and globally.

Speaking to the American newspaper during his working visit to New York, PM Lee did not mince words about what he saw as an "extreme menu of choices" currently on offer in the US polls.

"You believe in your system, with checks and balances, you can have somebody who is far from ideal become the president and the system will prevent him from doing harm.

"And so far, it has worked in the sense that you have had presidents who have not measured up, but after some time, well, the world comes back.

"But I do not think you have ever had such an extreme menu of choices as you have in this election, choices which are likely to end up on the ballot paper," he said, while making a passing mention of the regimes in wartime Europe.

"If you do, well, there are precedents. In Asia, you can write those off as unstable, immature democracies, but in Europe, before the war, such stress led to very extreme outcomes in Germany, in Italy and in the end, you have a very unhappy result."

And when asked about a litany of proposals made by US candidates on the campaign trail, PM Lee noted that many of them have the potential to cause serious harm to the reputation of the US.

Take for instance, the proposal from billionaire Donald Trump that the US enter into a trade war with China by imposing a 40 per cent tariff on Chinese goods being imported.

"It is hypothetical, but if you put that to any other country, those who have no choice would lie back and enjoy it. Those who have a choice will say, 'Chuck it, I cannot work on this basis'. It is not possible, because if that is how we work, seriously-entered-into undertakings can be just torn up and because the Americans are not happy, we can do it over again, then how do we conclude a new agreement? How do I know where the bottom line is?"

And the same goes for calls from Mr Trump and former secretary of state Hillary Clinton for the Trans-Pacific Partnership (TPP) free trade pact to be renegotiated.

"It has been with the President personally pushing every year and every opportunity has taken us since about 2010 until now to settle and to renegotiate and reopen, you are giving more or you are asking for more? And if you are asking for more, who is going to volunteer to give? Your candidates have to face election, the Japanese prime minister faces re-election, too."

The TPP, of which Singapore is a part of, was agreed to last year, although each of the 12 countries need to have the agreement ratified by their respective domestic legislatures before it can come into force.

That process is thought to be especially difficult in the US, and Mr Lee said he was not optimistic that it could be done.

Still, he stressed that the TPP was a critical anchor for the White House's Asia policy and said that the consequences of failure at this stage were drastic, both economically and geopolitically.

"You can say that you are rebalancing towards Asia, but is it words or is it deeds?" he asked.

"And even if, in fact, you are rebalancing towards Asia with aircraft carriers and aeroplanes, what is it in aid of?"

He added: "If America is not able to play the same game in the same league, and then your friends will say, I want to be friends with the US but to what extent can I depend on the US?

"And it is not just the small countries. I mean, the Japanese have gone out on a limb, Mr Abe, to commit, to participate in the TPP… And if at the end of it all, you let him down, which next Japanese Prime Minister is going to count on you, not just on trade, but security?"

And the TPP similarly has implications on the sort of role the US can play in the South China Sea issue.

"Without that (TPP), without the interest in a broad range of mutual cooperation, America is just another country which has some claim or some assertion," he said.

Despite all the challenges, he made it clear that the US continues to have an important leadership role to play and that was not about to change in the near future.

"You will still be the leader and if you do not cooperate, nothing can be done. If you do not do climate change, there is no climate change solution for the world because unless you are committed, we cannot get the Chinese along, much less the Indians and everybody else.

"So, your role remains indispensable. Whether you are prepared to step up to that, or whether you decide to chuck it and just ignore what the consequences will be, because they come after the current presidential term, well, that, we cannot say how your political process will turn out."

Mr Lee is in the US for a week-long visit.

On Friday (April 1), he will attend the 4th Nuclear Security Summit in Washington.