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

Democrats accuse Nunes of being too close to the president to be able to head the probe. Some of Nunes’ fellow Republicans have questioned his objectivity after he made a controversial announcement last week about U.S. spy agency surveillance.

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.”

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

The spectre of possible Russian influence on the election in Trump’s favour has cast a shadow over the president’s first two months in office.

Nunes announced last week without providing a source that he had information Trump associates may have been ensnared in incidental intelligence collection before the president took office in January.

The lawmaker acknowledged visiting the White House before making the announcement but has declined to say who he met there.

Critics say that Nunes’ disclosure was 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.

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Nunes, who was a member of the team that ran Trump’s transition to the presidency 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.”

But Republican senators John McCain and Lindsey Graham, influential foreign policy hawks, joined Democrats in questioning Nunes’ objectivity although they stopped short of calling on him to remove himself from the probe as Democrats have done.

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