US President Donald Trump's son-in-law, Jared Kushner, answered questions from senate investigators for hours behind closed doors on Monday, acknowledging four meetings with Russians during and after Trump's victorious White House bid and insisting he had "nothing to hide".

He emerged smiling to publicly declare, "All of my actions were proper."

Kushner, a quiet insider who avoids the spotlight, was the first top Trump lieutenant to be quizzed by the congressional investigators probing Russia's meddling in the 2016 presidential election.

The wealthy developer-turned-presidential adviser spoke privately with staff members of the Senate Intelligence Committee and will return to talk to the House intelligence panel on Tuesday.

"Let me be very clear," Kushner said afterwards in a rare public statement at the White House. "I did not collude with Russia, nor do I know of anyone else in the campaign who did so."

Trump watched on TV as Kushner made his appearance outside the West Wing and "thought Jared did a great job," said White House spokeswoman Sarah Huckabee Sanders. She said his House testimony on Tuesday would show "what a hoax this entire thing is."

Kushner's appearance has been highly anticipated, in part because of a series of headlines in recent months about his interactions with Russians and because until Monday he had not personally responded to questions about an incomplete security clearance form and his conversations with foreigners.

Earlier on Monday, Kushner released an 11-page statement that was billed as his remarks to both the Senate and House committees.

In it, he acknowledged his Russian contacts during the campaign and the following weeks, in which he served as a liaison between the transition and foreign governments.

His statement was the first detailed defence from a campaign insider responding to the controversy that has all but consumed the first six months of Trump's presidency.

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Kushner also denied reports that he discussed setting up a secret back-channel with Russian Ambassador Sergey Kislyak.

In the statement, Kushner described contacts with Kislyak and other Russian officials as normal in his former role as the campaign liaison to foreign governments.

"I did not collude, nor know of anyone else in the campaign who colluded, with any foreign government," he said in the statement.

Kushner is married to Trump's eldest daughter Ivanka.

He said he did speak with Kislyak in December at Trump Tower, but that conversation was about policy in Syria.

He also said he met a Russian banker, Sergey Gorkov, at the request of Kislyak, but no specific policies were discussed.

Trump Jr and Trump's former campaign manager, Paul Manafort, who was also at the June 2016 meeting, were scheduled to give evidence before the Senate Judiciary Committee this week.

But on Friday their attorneys said the two men were in discussions to be privately interviewed by staff or politicians, though the chairman of the committee, Iowa Senator Chuck Grassley, has said they will eventually testify in public.

Trump took to Twitter over the weekend to defend himself and repeat his criticism of the investigations.

On Sunday, he tweeted: "As the phony Russian Witch Hunt continues, two groups are laughing at this excuse for a lost election taking hold, Democrats and Russians!"