US President Donald Trump's top aides said on Thursday that Russia is behind "pervasive" attempts to interfere in upcoming US elections, in a rejection of the denials of meddling that Russian President Vladimir Putin made directly to Trump.

Trump's national security team, including intelligence director Dan Coats and national security adviser John Bolton, appeared in the White House briefing room to stress that a major effort is under way to protect the integrity of congressional elections in November and the 2020 presidential election.

Trump has publicly voiced skepticism about Russia's role in US election meddling, drawing accusations from Democrats and Republicans alike that he is ignoring a threat to American democracy.

"I have great confidence in my intelligence people, but I will tell you that President Putin was extremely strong and powerful in his denial today," Trump said after talks with Putin in Helsinki, Finland, on July 16.

We acknowledge the threat, it is real, it is continuing, and we're doing everything we can to have a legitimate election

But Coats, Bolton, FBI Director Christopher Wray, Homeland Security Secretary Kirstjen Nielsen and the director of the National Security Agency, Paul Nakasone, said Russia is to blame along with other foreign actors.

"Our focus here today is simply to tell the American people we acknowledge the threat, it is real, it is continuing, and we're doing everything we can to have a legitimate election," Coats said, adding: "It is pervasive, it is ongoing, with the intent to ... drive a wedge and undermine our democratic values."

The illegal activity includes criminal efforts to suppress voting and provide illegal campaign financing, cyber attacks against voting infrastructure, along with computer intrusions targeting elected officials and others, US officials said.

"In regards to Russian involvement in the midterm elections, we continue to see a pervasive messaging campaign by Russia to try to weaken and divide the United States," Coats said. "We will continue to monitor and warn of any such efforts."

Wray said the FBI has opened investigations into election interference.

"We also know the Russians tried to hack into and steal information from candidates and government officials alike," Coats said.

He said Russia was not the only country working to undermine American elections.

A federal special counsel is leading a criminal investigation of Russian interference in the 2016 US presidential election, and any possible cooperation with Trump's presidential campaign. Trump has insisted there was no collusion between his campaign and Russia in an attempt to sway the 2016 election in his favor.

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