On the spot as the FBI's new acting director, Andrew McCabe assured U.S. senators Thursday he will alert them to any effort to interfere with the investigation into Russia's election meddling and possible ties with Donald Trump's campaign.

The president's firing of FBI Director James Comey on Tuesday has led Democrats and others to raise concerns about the future of the investigation.

But McCabe, speaking publicly for the first time since his former boss's ouster, said there has been "no effort to impede our investigation."

"You cannot stop the men and women of the FBI from doing the right thing," he declared. He also said he would not inform the White House about developments in the probe.

He assured the committee the FBI had sufficient resources to conduct the investigation.

McCabe responded to questions from the top Democrat on the Senate intelligence committee, Mark Warner of Virginia, who said he thought Comey's dismissal was directly related to the Russia investigation.

McCabe, CIA Director Mike Pompeo, Director of National Intelligence Daniel Coats, NSA Director Admiral Mike Rogers; Defence Intelligence Agency Director Lt.-Gen. Vincent Stewart and National Geospatial-Intelligence Agency Director Robert Cardillo testify before the Senate intelligence committee. (Yuri Gripas/Reuters)

Days before he was fired, Comey requested more resources to pursue his investigation, U.S. officials have said, fuelling concerns that Trump was trying to undermine a probe that could threaten his presidency.

It was unclear whether word of the Comey request, put to Deputy Attorney General Rod Rosenstein, ever made its way to Trump. But the revelation intensified the pressure on the White House from both political parties to explain the motives behind Comey's stunning ouster.

Committee chairman Senator Richard Burr abruptly left the hearing with Warner to meet with Rosenstein, stirring speculation that the meeting signalled a development in the saga of why Trump fired Comey.

The media gathered outside the meeting, but Burr and Warner emerged, saying Comey's firing wasn't discussed.

Rather, the senators said, the meeting was about how the Senate intelligence committee probe into Russian activities during last year's election could proceed without interfering with the FBI's investigation into the matter.

During the hearing, McCabe also seemed to undermine recent White House claims that Comey had lost the confidence of the rank and file of the FBI as well as the public in general.

"That is not accurate," McCabe said in a response to a senator's question about the White House assertions. "I can tell you also that Director Comey enjoyed broad support within the FBI and still does to this day."

Trump says he was going to fire Comey anyway

In an interview with NBC News, Trump said he had planned to fire Comey before he met Monday with Rosenstein and Attorney General Jeff Sessions, contrary to earlier statements from the White House.

White House officials had said earlier in the week that Trump asked Sessions and Rosenstein for their opinions about Comey, and then Trump acted on those recommendations.

In announcing Comey's firing, the White House had circulated a scathing memo, written by Rosenstein, criticizing the director's handling of the Hillary Clinton email investigation.

Statement from <a href="https://twitter.com/WhiteHouse">@WhiteHouse</a> <a href="https://twitter.com/PressSec">@PressSec</a> on <a href="https://twitter.com/FBI">@FBI</a> Director <a href="https://t.co/EdBRntMim5">pic.twitter.com/EdBRntMim5</a> —@PressSec

Trump is the first president since Richard Nixon to fire a law enforcement official overseeing an investigation with ties to the White House. Democrats quickly accused Trump of using Comey's handling of the Clinton investigation as a pretext and called for a special prosecutor into the Russia probe. Republican leaders brushed off the idea as unnecessary.

Trump has denied any collusion between his election campaign and Russia.

Russian interference a 'direct assault'

Comey's firing and Russia were the central topics of Thursday's hearing, with questions about timing brought up by senators from both parties.

U.S. President Donald Trump had considered firing James Comey since he took office in January, a White House spokesperson said Thursday. (Pablo Martinez Monsivais/Associated Press)

Before testimony began, Warner said he had hoped to be able to question Comey about any Trump associates with ties to Russia, but his firing "cost us the opportunity to get at the truth, at least today."

He questioned how seriously the administration was taking any investigation in Russia.

"We will not be deterred from getting to the truth," Warner said.

Warner, a Democrat, also said Russia's interference in foreign elections is one of the top worldwide security threats.

"Russia's direct interference in democratic processes around the globe is a direct assault that we must work together to repel," Warner said.

Warner asked all the intelligence officials testifying whether a January report about Russian interference was accurate. The officials unanimously said it was.

Warner responded by confirming the report wasn't "fake news."

In response to a question from Senator Marco Rubio, the officials all agreed that interference in U.S. politics by Russia is continuing.

The officials offered few details on any specific threat posed by North Korea, saying that more detail would be given in a closed-door session that followed the morning's open session.