WASHINGTON -- Former CIA Director John Brennan has said he spoke directly to the head of Russia's main security agency, complaining about the harassment of U.S. diplomats and apparent Russian efforts to meddle in last year's election campaign.

Brennan told the House Intelligence Committee on May 23 that the conversation with Federal Security Service director Aleksandr Bortnikov occurred in August, as the U.S. presidential campaign was ramping up.

Brennan said he told Bortnikov that continued Russian interference would backfire and prevent any warming of relations between Moscow and Washington.

The same month that Brennan spoke to Bortnikov, the FBI opened an investigation into possible collusion between members of Trump's campaign and Russian officials.

Those ties are the focus of not only the FBI probe but several congressional investigations as well.

The issue has overshadowed President Donald Trump's administration since he took office in January.

Trump's firing of FBI Director James Comey earlier this month triggered a political uproar, and prompted the Justice Department to appoint a special counsel to oversee the agency's investigation into the Trump administration.