American intelligence and law enforcement officials have reportedly launched an investigation into what they believe is a wide-ranging covert Russian operation to disrupt the US presidential election by confusing voters, undermining faith in the electoral process and possible directly disrupting voting tabulations.

A key part of the operation, officials believe, is the use of hacking to spread confusion and disinformation, sources told the Washington Post.

"This is something of concern for the [the Director of National Intelligence James Clapper Jr]," said Charles Allen, a former long-time CIA officer who has been briefed on some of the issues. "It is being addressed."

It is not only hacks leading up to the November election that officials are concerned about, but possible tampering with the voting process itself, sources told the Post. The FBI has alerted state and local officials to potential cyberthreats, and the Department of Homeland Security is offering aid to help localities deal with cyber-disruptions on election day.

Besides the Democratic National Committee, Arizona, Illinois and both the Democratic and Republican parties have already been the victims of either attempted or successful cyberattacks that law enforcement experts are investigating.

Particularly vulnerable is electronic balloting from overseas. And cyberattacks could disrupt vote tabulations being transmitted to state-level offices.

Experts have already linked a number of hacks — including the 20,000 emails stolen from the Democratic National Committee and exposed on WikiLeaks — to operations they believe are being orchestrated by the Russian government.

"We've seen an unprecedented intrusion and an attempt to influence or disrupt our political process," said Democratic California Congressman Adam Schiff, referring to the DNC email leak on the eve of the Democratic convention. The disclosures forced the resignation of committee chairwoman Debbie Wasserman Schultz.

Russian President Vladimir Putin has denied any such operations. But he has also asked about the DNC leaks: "Does it even matter who hacked this data? The important thing is the content that was given to the public."

In a bizarre episode of the US presidential race, Republican candidate Donald Trump even challenged Russia to "find" unreleased emails — some of which could be classified — on rival Hillary Clinton's private server that were saved there while she worked as secretary of state. Trump later said he was just being "sarcastic".

US President Obama and Putin talked privately about cyber-spying and other issues during the G20 Summit in China. After their meeting Monday, Obama acknowledged tensions between the two nations over digital espionage.