Two officials in former President Barack Obama's administration will testify on Monday in a Senate investigation into allegations of Russian meddling in the 2016 U.S. election and possible collusion between President Donald Trump's campaign and Moscow.

James Clapper, the Director of National Intelligence under Obama, and Sally Yates, who was Deputy Attorney General, will testify to the Senate Judiciary subcommittee on crime and terrorism, the first such public testimony by former officials from the Democratic administration in one of congressional probes on Russia.

Congressional committees began investigating after U.S. intelligence agencies concluded that Russian President Vladimir Putin had ordered hacking of Democratic political groups to discredit the election and sway the voting towards Republican Trump, who won an upset victory in November.

Moscow has denied any such meddling. Trump also has dismissed the allegations, suggesting instead that Obama might have wiretapped his Trump Tower in New York or that China may have been behind the cyber attacks. No evidence has been found to support either allegation.

The public hearing will be the first featuring testimony by Obama administration officials who have left government. Trump fired Yates from the Department of Justice in January, and Clapper retired on Jan. 20, when Trump was inaugurated.

Senator Lindsey Graham, the subcommittee's chairman who called the hearing is a Russia hawk and sometime critic of Trump who has been one of the leading Republican voices calling for a thorough investigation of Russia and the election.

Yates is expected to tell the senators that on Jan. 26, when she was acting Attorney General, she had warned White House Counsel Don McGahn that then-National Security Adviser Michael Flynn had not told the truth about conversations he had with Sergei Kislyak, Moscow's Ambassador to Washington, about U.S. economic sanctions on Russia.

Flynn resigned after less than a month in office.

The congressional hearings have been shadowed by allegations, mostly from Democrats, that lawmakers are too partisan to investigate effectively.

In the lead-up to Monday's hearing, Susan Rice, who was Obama's national security adviser, declined an invitation to testify because it had come only from the Republican Grahamand not Senator Sheldon Whitehouse, the subcommittee's ranking Democrat.

Her refusal was first reported by CNN.

Rice's name was linked to the Russia investigation when Trump suggested she might have broken the law by asking intelligence analysts to reveal the name of a Trump associate mentioned in an intelligence report.

She denied doing anything inappropriate, and there is no evidence to substantiate Trump's allegation.

Trump tweeted on Thursday that it was "Not good!" that Rice had not agreed to testify.

The probe being led by Graham and Whitehouse is one of three main congressional investigations of Russia and the 2016 U.S. election. The FBI and U.S. intelligence agencies are conducting separate investigations.

Clapper, Yates and another official who served under Obama, former CIA Director John Brennan, had been scheduled to testify to the House of Representatives intelligence committee in March, but that hearing was cancelled by the panel's chairman, Republican Devin Nunes.

Nunes, a Trump ally, has since recused himself from the Russia investigation amid concerns that he was too close to the White House to lead a credible probe.

Yates, Clapper and Brennan are now due to appear at a public hearing of the House committee that has not been scheduled.