USA intelligence agencies concluded in January that Moscow tried to tilt the election campaign in Trump's favour, including by hacking into the emails of senior Democrats.
News reports that Trump asked his national intelligence director and National Security Agency chief to state publicly there was no evidence of collusion have heightened criticism. Last week, Deputy Attorney General Rod Rosenstein appointed former FBI Director Robert Mueller to act as a special counsel to investigate any possible collusion.
Brennan's interlocutor denied any Russian interference but said he would pass on the warning to President Vladimir Putin, the ex-CIA chief said.
The Senate intelligence committee says it will subpoena two of former national security adviser Michael Flynn's businesses.
Former CIA director John Brennan has said that he warned the head of the Russian intelligence services against interfering in the 2016 U.S. president election in an August phone call.
He said that Americans don't always know when they're helping Russian spies because Russian intelligence agents often use intermediaries.
"Frequently, individuals who go along a treasonous path do not even realize they're along that path until it gets to be a bit too late", Brennan said of how Russian Federation typically recruits people in the U.S.to work on its behalf.
The former Central Intelligence Agency director warned that he does not believe the Russians are done trying to interfere in America's elections and could try to influence the 2018 congressional elections.
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The letter says that the current context of the Senate's investigation into Russia's meddling in the 2016 election threatens that "any testimony he provides could be used against him".
Under questioning from Rep. Ileana Ros-Lehtinen, R-Fla., Brennan said the Russians have been trying to disrupt Western elections since the 1960s, and that they've quickly adapted to the times. "Mr. Putin and Russian officials are trying to actively exploit what is going on now in Washington to their benefit and to our detriment".
On another intelligence matter, Brennan said that while he was CIA director he shared classified information with Russian Federation and other nations about threats related to terrorism.
Trump, now on a nine-day global trip, has had his own conversations with the Russians questioned in light of reports that he shared extremely classified intelligence with Russian diplomats in the Oval Office on May 10.
Trump has defended his disclosure, saying he had the right to share the information.
First, he said, such intelligence is not shared with ambassadors but rather through intelligence channels. Brennan said he's studied Russian intelligence activities over the years, and how Russian intelligence services have been able to get people to betray their country.
Brennan told how he called the head of the Russian intelligence service, the FSB, on August 4 of past year.
"I said American voters would be outraged by any Russian attempt to interfere in our elections", Brennan said.
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However, he blamed the uncertainty over his future for contributing to the club failing to qualify for the Champions League. It added: "KSE is a committed, long term investor in Arsenal and will remain so".
He said he had personally told a senior Russian security official that continued meddling would backfire and prevent any warming of relations after the election.
"It should be clear to everyone Russian Federation 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", Brennan said.
Brennan told the lawmakers that he had made contact with his Russian counterpart, head of Russia's Federal Security Service Alexander Bortnikov, in August and warned him that any interference in the US presidential election would "destroy any near-term prospect" of improved relations between the USA and Russia.
The former CIA director will testify before a House intelligence committee at 10 a.m. ET today.
Brennan made the comments on Tuesday during a hearing with the House of Representatives Intelligence Committee.
Brennan's testimony before the House intelligence committee on Tuesday comes one day after Flynn invoked his constitutional right against self-incrimination in response to a subpoena from the Senate intelligence committee.
Reuters reported last week that Trump's fired former national security adviser Michael Flynn and other advisers to Trump's campaign were in contact with Russian officials and others with Kremlin ties in at least 18 calls and emails during the last seven months of the presidential race.
He also said that before such sharing, the USA would go back to the intelligence partner that provided the information to make sure it would not compromise operatives. Brennan said it was possible for someone to be helpful to the Russian cause without realizing it.
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