The head of a congressional committee investigating contacts between Donald Trump's campaign and Russia said on Monday the panel had not seen evidence of inappropriate communications, prompting the panel's top Democrat to insist it was too early to make such a determination.

Devin Nunes, Republican chairman of the House Permanent Select Committee on Intelligence, also said there was no need for a special prosecutor and dismissed a suggestion that Trump should release his tax returns to clear up allegations he has business ties to Russia.

"What are we going to appoint a special prosecutor to do, exactly?" he asked reporters.

Nunes, who was a member of Trump's presidential transition team, said U.S. intelligence officials had not yet presented the committee with evidence of contacts between Trump campaign staff and Russian intelligence.

"It's been looked into and there's no evidence of anything there," Nunes told a news conference, called after a weekend report by the Washington Post that the Trump administration asked him and Senate Intelligence Committee Chairman Richard Burr to call journalists to knock down reports about possible collusion.

The story fuelled concerns about whether congressional committees led by Trump's fellow Republicans would conduct a serious investigation of the politically charged allegations.

Nunes acknowledged that the White House had given him a reporter's number, but said the administration had not asked him to knock down reports. He said his communications with news organizations was intended to promote transparency.

Underscoring the partisan divide, Representative Adam Schiff, the intelligence committee's top Democrat, told his own news conference later on Monday that a non-partisan investigation would be most effective, insisting it was too early to comment on any evidence.

"When you begin an investigation, you don't begin by stating what you believe to be the conclusion," he said.

'STONEWALLING'

House Democratic leader Nancy Pelosi said Nunes' remarks raised "serious questions about stonewalling."

Potential contact between Trump's presidential campaign and Russia, and possible Russian attempts to influence the 2016 election on Trump's behalf, have prompted Democrats to demand a select committee or special prosecutor.

Most of Trump's fellow Republicans in Congress have resisted such suggestions, prompting Democrats to contrast their approach with their multiple investigations of 2016 Democratic presidential candidate Hillary Clinton, including her use of a private email server.

Republican Representative Darrell Issa broke with the party line by calling for an independent review. "I want the Trump administration to be successful and that starts with embracing high standards for openness and transparency," he said in a statement.

Schiff said he was not confident that James Comey, the director of the Federal Bureau of Investigation, would provide investigators with all the information they would need.

Comey, a Republican, drew furious criticism from Democrats for saying just before the election that he was looking at emails related to Clinton's use of a private server.

Nunes said he did not want U.S. citizens to be hauled before Congress because of news reports about their potential ties to Russia. "We can't have McCarthyism back in this place," he said, alluding to the notorious 1950s Senate hearings into Americans' potential ties to Communism.