One of America's most powerful Republicans, Senator John McCain says Russian President Valdimir Putin is more threatening than Islamic State for destroying the fundamentals of democracy.

Senator McCain told ABC's 7.30 program on Monday night that Mr Putin "is the premier and most important threat, more so than ISIS".

"I think ISIS can do terrible things, and I worry a lot about what is happening with the Muslim faith and I worry about a whole lot of things about it, but it is the Russians who are trying - who tried to destroy the very fundamentals of democracy and that is to change the outcome of an American election," he said during his stint as a guest of the US study centre at the University of Sydney

"I have seen no evidence they succeeded but they tried and they are still trying. They just tried to affect the outcome of the French election."

He said the US needed to respond with increased sanctions against Russia for trying to influence the presidential election in November.

Senator McCain says China can restrain North Korea from using nuclear weapons.

"I don't think it's acceptable for the United States of America to have an intercontinental ballistic missile or a missile aimed at Australia with a nuclear weapon on it and depend on our ability to counter it with an anti-missile capability," he says.

He says he feels nervous "from time-to-time" with US President Donald Trump at the helm of the free world.

"I do believe that the president has great confidence in his national security team. I do believe that most of the time he accepts their advice and counsel," he says.

But Senator McCain says he reacts to what the president does, not what he says.

His comments come as German Chancellor Angela Merkel said the US was no longer a reliable partner for her nation or Europe.

"When America abandons its leadership then bad things happen and that vacuum is filled which evil influences. So I appreciate the European's viewpoint and their repudiation of Donald Trump but my friends, why don't they say, 'Maybe we shouldn't have given Barack Obama the Nobel Peace Prize before he was ever even elected.'"

It comes as the Australian government announced on Monday it would send another 30 troops to Afghanistan at America's request, boosting troop numbers to 300.

Australia's Foreign Minister Julie Bishop said Australia and the US shared "one of the closest intelligence-sharing relationships that one could imagine".

"The United States is indispensable and the vital for Australia's national security and the security and safety of our citizens," she says.