The chairman of a US congressional committee on intelligence said Sunday that he has seen "no evidence" that President Donald Trump's campaign colluded with Russia during the runup to the 2016 election.

Based on "everything I have up to this morning -- no evidence of collusion," said US Representative Devin Nunes, head of the House Intelligence Committee, speaking to the Fox News Sunday television program.

He made his remarks one day before FBI director James Comey is due to appear before his panel, facing lawmakers seeking answers about Trump's potential Russia ties and the president's extraordinary accusation of wiretapping by his predecessor.

Nunes added that he agreed with the president's accusation that there are elements inside the intelligence community or FBI leaking information to undercut the Trump presidency.

"It's pretty clear that that's happening," he said.

"There's even been stories written about it in numerous newspapers talking about how they said they left breadcrumbs around to hurt the Trump administration."

But he suggested that many of the leakers may now be gone.

"I think it was largely people maybe who were there, had classified information, who are now no longer there and decided to leak it," he said.

"Clearly to leak Michael Flynn's name talking to the Russian ambassador," Nunes said. "That was clearly designed to hurt Gen. Flynn and the president's national security adviser."