The computer network used by the campaign of Democratic presidential nominee Hillary Clinton was hacked as part of a broad cyberattack on Democratic political organisations.

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The latest attack, which was disclosed to Reuters on Friday, follows reports of two other hacks on the Democratic National Committee, or DNC, and the party's fundraising committee for candidates for the US House of Representatives.

A Clinton campaign spokesman said on Friday an analytics data program maintained by the DNC and used by the campaign and several other entities "was accessed as part of the DNC hack".

"Our campaign computer system has been under review by outside cybersecurity experts. To date, they have found no evidence that our internal systems have been compromised," said Clinton campaign spokesman Nick Merrill.

The US Department of Justice national security division is investigating whether cyberattacks on Democratic political organisations threatened US security, sources familiar with the matter said on Friday.

The involvement of the Justice Department's national security division is a sign the Obama administration has concluded the hacking was sponsored by a state, people with knowledge of the investigation said.

Hackers, whom US intelligence officials have concluded were Russian, gained access to the entire network of the fundraising Democratic Congressional Campaign Committee, or DCCC, said people familiar with the matter, detailing the extent of the breach to Reuters for the first time.

Cybersecurity experts and US officials said this week they had concluded, based on analysis of malware and other aspects of the DNC hack, that Russia engineered the release of hacked Democratic Party emails to influence the US presidential election.

Russian officials could not be immediately reached for comment.

After emails were leaked from the DNC hack, Republican presidential nominee Donald Trump on Wednesday invited Russia to dig up thousands of "missing" emails from Clinton's time at the State Department, prompting Democrats to accuse him of urging foreigners to spy on Americans.

On Thursday Trump said his remarks were meant as sarcasm.

In Washington the DCCC said it had hired cybersecurity firm CrowdStrike to investigate."We have taken and are continuing to take steps to enhance the security of our network," the DCCC said.