WASHINGTON (Reuters) - The top Republican in Congress on Tuesday stood by Devin Nunes, the head of the House of Representatives intelligence committee who is under fire over his handling of an investigation into possible Russian ties to President Donald Trump’s election campaign.

Democrats and some prominent Republicans have criticized Nunes, a Trump ally, since he announced last week that U.S. security agencies had collected information on Trump’s transition team while conducting legal surveillance on other targets.

Democrats see the surveillance accusations, which Nunes said were based on unspecified intelligence reports, as a distraction from the probe into possible Russian influence on last year’s election in Trump’s favor that has cast a shadow over the president’s first two months in office.

They have demanded Nunes step aside from his committee’s investigations into the Russia ties.

House Speaker Paul Ryan, asked at a news conference whether Nunes should step aside from the investigation and if he knew the source of Nunes’ claims about surveillance, said: “No and no.”

Trump, at an event in the White House, declined to comment on whether Nunes should step back.

Nunes had acknowledged visiting the White House grounds to meet a source and review the intelligence reports before making his announcement that Trump and his associates may have been ensnared in incidental intelligence collection.

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However, he told ABC News on Tuesday he would not divulge who gave him intelligence reports, even to other members of his panel, saying: “We will never reveal those sources and methods.”

Critics say Nunes’ statements were an effort to justify Trump’s unfounded accusations this month that his Democratic predecessor, Barack Obama, had directed surveillance on Trump Tower during the election campaign.

The Senate Intelligence Committee, which is also investigating Russia’s role in the election, wants to question Trump’s son-in-law and adviser, Jared Kushner, about meetings he held with the Russian ambassador and a Russian banker in December.

Nunes, who was a member of Trump’s transition team after the Nov. 8 election, told reporters on Tuesday the House panel’s investigation was moving forward. Asked whether he would recuse himself, he said: “The investigation continues.”

Ryan spokeswoman AshLee Strong said in an email that “Speaker Ryan has full confidence that Chairman Nunes is conducting a thorough, fair, and credible investigation.”

However, Republican senators John McCain and Lindsey Graham, both influential foreign policy hawks, questioned Nunes’ actions.

“I think he put his objectivity in question, at the very least,” Graham said on NBC’s “Today” show.

One Republican lawmaker, Representative Walter Jones of North Carolina, went so far as to join Democrats in demanding a special committee to investigate the Russia probe, telling Fox News on Tuesday that Nunes was now “tainted.”