Presumptive Republican presidential nominee Donald Trump's temperament makes him a "dangerous proposition" to be the US commander-in-chief and his ideas could put the world on the path of a nuclear arms race, Democratic front-runner Hillary Clinton's close aide has said.

"It is very, very difficult to pin down where he stands on a lot of these policies," Clinton's Senior Foreign Policy Advisor Jake Sullivan said when asked about his views on Trump's foreign policy proposals during a discussion at the Asia Society here yesterday.

In the conversation with former Australian Prime Minister Kevin Rudd on foreign policy, Sullivan slammed Trump for his continuously oscillating views on how he would deal with other nations, including major powers like Russia and China, saying he does not have the temperament to be an effective commander-in-chief.

"He'll say on the one hand, the Chinese are eating our lunch, and on the other hand, we have all the leverage in the world to make the Chinese do whatever we want," Sullivan said.

"On the one hand, we should sit down with the Russians ...on the other hand, if I need to, I'll just shoot down Russian fighter jets. He says on the one hand, the United States can do whatever it wants, wherever it wants, to achieve whatever purpose it wants. On the other hand, we're doing too much and we can't do all that. We have to stop," Sullivan said.

Sullivan, who has also served as a national security adviser to Vice President Joe Biden, said Trump's world views can put America at great risk.

Sullivan said he views Trump as someone "unlike any candidate that we have seen before, who looks to be close to securing the nomination of a major party, in that the ideas he's putting forward and the temperament he has displayed behind them make him a dangerous proposition to be US commander-in-chief."

He said Trump's support on various occasions for nations getting more nuclear weapons can spark an arms race, in turn increasing the chances of such weapons falling in the hands of terrorists.

Trump's views that more countries should be able to obtain nuclear weapons has the "very real risk of sparking a nuclear arms race, and also makes it increasingly likely that terrorists will get their hands on nuclear weapons, which is the greatest threat that the United States faces", Sullivan said.

He also slammed Trump for his views that America should order its military officers, "against the law, to kill the families of terrorists".

"He has said many things along those lines. When you add up the totality of this, however you slice it, whichever pieces you accept or discard, the picture that is painted is one of a tremendously dangerous risk," Sullivan said.

In a statement, the Clinton Campaign said nobody is better at proving Trump's foreign policy would be disastrous than the 69-year-old tycoon himself.

"With one short interview on ITV's Good Morning Britain, Trump set off a media firestorm in the UK, stoking fears about the potential fraying of our decades-long special relationship, and what that would mean for the UK's national security and standing in the world," the campaign said.

"Trump continues to demonstrate that his reckless, shoot-from-the-hip approach to foreign policy would make the country and the world far less safe. He casts aside our most important allies when their leaders criticise his dangerous proposals, and heaps praise on our adversaries if they say a kind word about him," it said.

"Simply put, this is not a man who has the temperament or the steady hand to be our commander-in-chief. Check out the reaction in the UK to Trump's alarming comments about our ally, as well as the latest updates on his love affair with Putin below," the Clinton Campaign alleged.

Sullivan, while talking about a future Hillary Clinton administration, said it would like to see political stake by China to help bring about stability in Afghanistan.

He stressed that a strong Beijing that is "playing by the rules" is very much in the interests of the US.

"An area of substantial overlapping between the US and China is how we think about this problem of terrorism and violent extremism and trying to see how the Chinese can take on a more meaningful role in helping deal with the problem of terrorism and violent extremism," Sullivan said.

He said a Clinton administration would like to see not just an economic stake by China in Afghanistan but also a political one where it helps in bringing about stability in the war-torn nation.

"On the issue of bringing long term stability in Afghanistan, a country very much in China's neighbourhood, in which China has a stake economically and in which we would like China to also have a stake politically, for them to have a political commitment to helping bring about a better future for Afghanistan," he said.

In the conversation with Rudd on Clinton's foreign policy priorities, Sullivan said Clinton is "very much committed" to a deep relationship with Beijing and "a strong, thriving China that is playing by the rules and is participating responsibly in the international order is very much in the interests of the US."

As a country that is gaining in power and influence in the world, Sullivan said China has a "deeper stake" in a stable, secure and "rules-based" order and "therefore has some responsibility" towards it.

Beijing should have greater means to help "contribute to the protection and advancement of that order," Sullivan said.

"The country, because of the historic challenge posed by rising powers, also has a special responsibility to reassure its neighbours and others of its intentions," he said.

Sullivan emphasised that Beijing has "not fully lived up to all" of its responsibility in the international system "but I don't mean that as a statement of lecture but that has to be a part of the conversation going forward".

He stressed that whether it is the G20 or a future economic architecture for the Asia Pacific, "China has an incredibly important role to play and the more that we can get on the same page for what the common rules for the road are," the better it will be for the two countries as well as for the region.

Asked if whether the US and China are "enemies for life", he said such a scenario would be a "catastrophe" not only for the two nations but for the whole world.

"US and China have the wherewithal with responsible stewardship to build a cooperative, positive, constructive relationship that serves the interests of both countries, the region and the world. That does not mean it won't have a mix of cooperation and competition," Sullivan said.

"It does not mean that there won't be times that the US takes a firm line on activities the that we think are wrong that the Chinese are undertaking. It does not mean that there will not be tensions over certain things as we go foreword but a deep fundamental commitment to get a US-China relationship right has to be one of the core principles of the American foreign policy," he said.

He also underscored that getting the US-China relationship right means the two find a way to work together effectively to advance their own interests but also manage the relationship in a way that benefits everybody.