Russia "brazenly interfered" in the 2016 US election process despite the head of its FSB security service being warned it would hurt US-Russia ties, according to John Brennan, the former CIA director.

Brennan, while testifying at a hearing of the US House of Representatives Intelligence Committee on Tuesday, said it became clear last summer that Russia was attempting to interfere in the election.

"It should be clear to everyone Russia brazenly interfered in our 2016 presidential election process and that they undertook these activities despite our strong protests and explicit warning that they do not do so," he said.

Brennan said he believed he was the first US official to raise the matter of election interference with the Russians, citing a phone conversation he had in August last year with FSB head Alexander Bortnikov.

He said he raised published news media reports of Russian attempts to meddle in the election with the Russian official, who denied any involvement by Moscow.

Brennan said he briefed then President Barack Obama and other top officials, and that he discussed the matter with both Republican and Democratic US congressional leaders in August and September.

US intelligence agencies concluded in January that Russia tried to tilt the November presidential election campaign in Republican Donald Trump's favour, including hacking into and leaking emails of senior Democrats.

Russia has always denied the allegation.

Al Jazeera's Kimberly Halkett, reporting from Capitol Hill in Washington, DC, quoted Brennan as saying there was sufficient evidence to warrant the many investigations taking place right now.

READ MORE: Trump condemns probe by special counsel for Russia

Republicans on the committee pushed Brennan about whether there was evidence of collusion between Russia and the Trump campaign.

Brennan said the CIA focuses on intelligence, not "evidence", and he was not able to answer that question.

The testimony came the day after Michael Flynn, Trump's former national security adviser, invoked his constitutional right against self-incrimination.

The FBI is also conducting a counterintelligence investigation into Russia and the Trump campaign.