Russian President Vladimir Putin is a bigger threat to world security than the Islamic State group, US senator John McCain has told the ABC.

The Republican also admitted in an exclusive interview with 7.30 that President Donald Trump sometimes made him "nervous".

During a visit to Canberra, Senator McCain said Mr Putin was the "premier and most important threat, more so than ISIS".

"I think ISIS can do terrible things. But it's the Russians who tried to destroy the fundamental of democracy and that is to change the outcome of an American election," he said.

"I've seen no evidence they succeeded, but they tried and they are still trying to change elections.

"They just tried to affect the outcome of the French election. So I view Vladimir Putin — who has dismembered Ukraine, a sovereign nation, who is putting pressure on the Baltics — I view the Russians as the far greatest challenge that we have."

He said the US needed to respond to Russia with sanctions.

"We have done nothing since the election last November to respond to Vladimir Putin's attempt to change the outcome of our elections. So, way to go Vladimir. We haven't responded at all," he said.

"Hopefully when we get back from recess the Senate will enact sanctions on Russia."

Here's some of his thoughts on other topics:

Nervous about Trump

Sorry, this video has expired John McCain says Donald Trump does make him 'nervous from time to time'.

When it was put to him that some people feel nervous about international security with Mr Trump as President, Senator McCain said he understood why.

"I am nervous from time to time," he said. "I do believe that the President has great confidence in the national security team. I do believe most of the time that he accepts their advice and counsel.

"Can I tell you that he does [that] all the time? No. Does it bother me? Yes, it bothers me.

"I don't think there's any doubt that this FBI issue and the whole issue of the Russians, it's a scandal of significant proportions and it's going to be with us for quite a while.

"I hope we can separate that issue and all its ramifications from the need for us to win in Mosul, to win in Afghanistan, win in Raqqa and beat back this threat to our nations' national security, which we just saw a manifestation of the problem in Manchester."

Kushner allegations

The former presidential candidate visited Parliament during Question Time on Monday. ( ABC News: Marco Catalano )

On allegations that White House adviser Jared Kushner discussed a secret communication channel with Russia, Mr McCain said: "I don't like it, I just don't."

"I know some administration officials are saying this is standard procedure. I don't think it's standard procedure prior to the inauguration of the President of the United States by someone who is not in an appointed position.

"And I think Mr [James] Comey (the former FBI director) we now know took action that he did in regards to then-candidate Clinton in regards to false news that was being put out by the Russians.

"This is becoming more and more bizarre, in fact you can't make it up."

Paris agreement

With Mr Trump failing to commit to remain a part of the Paris agreement on curbing carbon emissions at last week's G7 meeting in Italy, some European leaders are concerned the US will pull out.

"I think the Europeans are legitimately concerned," Senator McCain said.

"I'd also point out that Barack Obama made that agreement without an agreement from the Congress of the US, which then allows this President to revoke it.

"Barack Obama made a major, major commitment without consultation with the Congress of the United States."

North Korea threat

Senator McCain also said North Korea had the potential to become a "very serious crisis along the lines of the Cuban missile crisis, unless we do everything we can to restrain North Korean behaviour".

"The key is China. China can restrain North Korean behaviour," he said.