Donald Trump's top national security chiefs say 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 the US President.

Key points: Officials say there is a pervasive messaging campaign by Russia to try to weaken and divide the US

Officials say there is a pervasive messaging campaign by Russia to try to weaken and divide the US The cyberthreat now exceeds the danger of a physical attack on the US, they say

The cyberthreat now exceeds the danger of a physical attack on the US, they say John Bolton says Donald Trump "leading unprecedented action to punish Russia"

Mr Trump's 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 was underway to protect the integrity of congressional elections in November and the 2020 presidential election.

The President has publicly voiced scepticism about Russia's role in US election meddling, drawing accusations from Democrats and Republicans alike that he was 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," Mr Trump said after talks with Mr Putin in Finland on July 16.

But Mr Coats, Mr Bolton, FBI Director Christopher Wray, Homeland Security Secretary Kirstjen Nielsen and the director of the National Security Agency Paul Nakasone, said Russia was 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," Mr Coats said.

"We continue to see a pervasive messaging campaign by Russia to try to weaken and divide the United States.

"It is pervasive, it is ongoing, with the intent to … drive a wedge and undermine our democratic values."

"Our democracy is in the crosshairs," Ms Nielsen said, adding that the cyberthreat now exceeded the danger of a physical attack against the US by a hostile foreign group.

Mr Wray said the FBI had opened investigations into election interference.

Donald Trump 'leading action to punish Russia'

The illegal activity included 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.

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

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

Sorry, this video has expired Donald Trump says he holds Vladimir Putin personally responsible for election meddling.

Mr Bolton also wrote to Senate Democrats, saying Mr Trump "has not and will not tolerate interference in America's system of representative government".

He said Mr Trump was "leading unprecedented action to punish Russia" for its efforts to disrupt 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 Mr Trump's presidential campaign.

Mr Trump has insisted there was no collusion between his campaign and Russia in an attempt to sway the 2016 election in his favour.

AP/Reuters