The US House of Representatives intelligence committee will investigate allegations of collusion between Donald Trump's presidential campaign and Russia, the top Democrat on the panel has said.

"We have reached a written agreement, the minority and the majority in the House intelligence committee, that we will investigate allegations of Russian collusion with the Trump campaign," Democratic Representative Adam Schiff told MSNBC on Wednesday.

US intelligence analysts have concluded that Russia tried to help Trump to win the White House by discrediting Democratic presidential nominee Hillary Clinton and her party through cyber attacks.

President Barack Obama, a Democrat, expelled Russian diplomats in retaliation in December.

Trump has denied any of his associates had contacts with Moscow before last year's election and dismissed the controversy as a "scam" perpetrated by a hostile news media.

Moscow has denied the accusations.

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Representative Devin Nunes, the Republican chairman of the House Permanent Select Committee on Intelligence, said on Monday that US intelligence officials had not yet presented the panel with evidence of contacts between Trump campaign staff and Russian intelligence.

Nunes was a member of Trump's presidential transition team.

The committee, which has been probing contacts between Trump's campaign and Russia, said in a statement that Nunes and Schiff had agreed on a classified six-page document laying out the scope of their investigation.

It said one question they would seek to answer was whether the Russian actions included "links between Russia and individuals associated with political campaigns or any other US persons".

The statement did not refer specifically to the Trump campaign.

Earlier on Wednesday, Nunes told Fox News that the committee would receive a briefing from intelligence officials on Thursday.

Trump fired his first national security adviser, Michael Flynn, last month for misleading Vice President Mike Pence over his conversations with Russia's ambassador to the United States.

Separately, then-US Senator Jeff Sessions spoke twice last year with Russia's ambassador, encounters he did not disclose when asked during his confirmation hearing to become attorney general about possible contacts between Trump's campaign and Russian officials, The Washington Post reported on Wednesday, citing justice department officials.

One of the meetings was a private conversation between Sessions and Russian Ambassador Sergey Kislyak which took place in September in the senator's office, the Post reported.