US security and intelligence agencies fear Russia-backed cyber-attack on election day: Report

US security and intelligence agencies fear that Russia-backed hackers might try another round of cyber-attack on the election day next week, multiple media reports have said.

Even though such a move is unlikely to impact the election outcome, but it might sow doubts about the legitimacy of the results, reports said.

"The assessment reflects widespread concern among US spy agencies that a months-long campaign by Russia to rattle the mechanisms of American democracy will probably continue after polls close on one of the most polarizing races in recent history, extending and amplifying the political turbulence," The Washington Post said in a major news story.

"I think its correct to say the Russians don’t think they can dictate the outcome," Congressman Adam B Schiff, the top Democrat on the House Intelligence Committee, was quoted as saying in the US media.

Still Russian intelligence services are likely to be "looking through their troves of hacked documents and seeing what they can release," he said.

In another news report, NBC News said government officials believe hackers from Russia or elsewhere may try to undermine Tuesday's presidential election. They are mounting an unprecedented effort to counter their cyber meddling.

According to the news channel, Russia has been warned that any effort to manipulate the actual voting or vote counting would be viewed as a serious breach.

"The Russians are in an offensive mode and working on strategies to respond to that, and at the highest levels," Michael McFaul, the US ambassador to Russia from 2012 to 2014, was quoted as saying in the US media.

The Russians "want to sow as much confusion as possible and undermine our process in ways they've done elsewhere," a senior Obama administration official told NBC news.

"So this is to make sure that we have all the tools at our disposal and that we're prepared to respond to whatever it is that they do," the official said.

"We need to be prepared on every front, not just technical but messaging, and so on. Because any reporting irregularity could be incredibly disruptive. They can cause tremendous chaos, and by the time we are able to attribute, the damage may have already been done," the official said.

US officials have said that given the nature of decentralization of elections, it would be extremely difficult for a nation or a non-state actor to alter ballot counts.

"This assessment is based on the decentralized nature of our election system in this country and the number of protections state and local election officials have in place. States ensure that voting machines are not connected to the Internet, and there are numerous checks and balances as well as extensive oversight at multiple levels built into our election process," the official said.