The top Democrat on the US House of Representatives' intelligence committee says Russian interference "unquestionably" had an influence on the 2016 US presidential election.

California congressman Adam Schiff made the remark in an interview with The Associated Press on Tuesday.

Schiff also commented on a former foreign policy adviser to President Donald Trump's campaign who has acknowledged in testimony to Congress that he had contact with a high-level Russian official while on a trip to Russia last year.

The congressman also urged Republican and Democrat leaders to support special counsel Robert Mueller in his investigation.

Washington DC - 7 November 2017

1. SOUNDBITE (English) Adam Schiff, Democratic member of the US House of Representatives and ranking member of the House Intelligence Committee: ++VARIOUS ANGLES++

"I think unquestionably it had an influence on the election. It influenced the election by millions and millions of Americans seeing Russian content on social media pushed out by the Kremlin. It had an effect in that the Russians were dumping emails that forced Hillary Clinton on a daily basis to have to defend herself and the campaign, and it changed the topic of the conversation on the campaign trail. But again the very best proof that the Trump campaign viewed the Russian interference as influential is the fact that the president used it as a central part of his campaign speeches on a daily basis."

2. Cutaway of audience

4. SOUNDBITE (English) Adam Schiff, Democratic member of the US House of Representatives and ranking member of the House Intelligence Committee:

"And the single greatest impediment to getting to that consensus is the President of the United States, who refuses to admit it even happened, who to this day calls it a hoax and a witchhunt. So he is I think quite deliberately for his own reasons preventing the country from coming together around a defence to further Russian meddling."

5. Wide of Schiff and AP Washington Bureau Chief Julie Pace

6. SOUNDBITE (English) Adam Schiff, Democratic member of the US House of Representatives and ranking member of the House Intelligence Committee: ++VARIOUS ANGLES++

"First, it was apparent right from the very beginning of the hearing that what Carter Page (foreign policy adviser to Trump's 2016 presidential campaign) had been saying publicly was completely at odds with what he would later testify to. And the documentary evidence that we were already in possession of so publicly even to up to two days before coming before Congress he was saying that when he went to Russia it was as a private citizen, he had no meetings of any consequence. It was basically talking to your man on the street, but as now has been revealed when we released the transcript, he met with people as high as the deputy prime minister of Russia."

(AP Washington Bureau Chief Julie Pace: "Which he's denied previously.")

"Which he denied previously, even at the very beginning of his interview I asked him: 'Did you have any private conversations any private meetings with Russian government officials?' (His response was) 'No, absolutely not.' But he sends a memo back to the campaign - which in itself is significant, because he was claiming this was a private trip - sends a memo saying that in a private conversation, basically, that I had with the deputy prime minister and others in the presidential administration and members of the Duma I gained important insight and outreach that I want to share with the campaign. Completely at odds with what you have been saying publicly."

7. Wide of Schiff and Pace

8. SOUNDBITE (English) Adam Schiff, Democratic member of the US House of Representatives and ranking member of the House Intelligence Committee: ++VARIOUS ANGLES++

"The most important thing I think that we can do right now is speak out on a very bipartisan basis that the President needs to leave (special counsel) Bob Mueller alone and finish this work, and I think that's the most important because we don't want to wait till there's a constitutional crisis. We need the White House to get the message now that that there will be a dramatic pushback should he try to fire Bob Mueller."

9. Audience member asking question

10. SOUNDBITE (English) Adam Schiff, Democratic member of the US House of Representatives and ranking member of the House Intelligence Committee:

"Probably the lowest hanging fruit is we're going to require that the tech companies, when they do political advertising, carry the same kind of disclaimer that other media are required to carry. So a paid ad on Facebook is going to need to disclose that it was paid for by what campaign then the candidate endorses the message. They were a carve out. They're not going to be carved out anymore."

11. Schiff and Pace