The FBI is investigating another hack at Democratic Party headquarters after it emerged that donors may have had their personal details stolen from a campaign fundraising database.

Sources believe the attack on the Democratic Congressional Campaign Committee (DCCC) may have been used to gather information about party backers, rather than to steal their money.

Cyber attackers set up a fake website for donations from what appears to have been a Russian IP address and then harvested donors' details, sources said.

The hack is another embarrassment for the party after internal emails and voicemails obtained by WikiLeaks revealed how senior officials and powerful donors were biased against Bernie Sanders.

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The FBI is investigating another hack at Democratic Party headquarters after it emerged that donors may have had their personal details stolen from a campaign fundraising database (file picture)

Four people familiar with the latest attack said the DCCC hack could be linked to the previous incidents, which many have blamed on Russia.

It is not clear what data was exposed, but donors routinely hand over credit card details, email addresses and phone numbers when making a contribution to the party.

It is also unclear if stolen information was used to hack into other systems.

The DCCC raises money for Democrats running for seats in the House of Representatives.

Two sources believe the information was first illegally accessed and then stolen in June, but it is only coming to light now following the WikiLeaks disclosures, two sources said.

In June, a bogus website with a similar address and design to the DCCC site was set up - leading donors to hand over money to the wrong people, according to the sources.

Cyber attackers set up a fake website for donations from what appears to have been a Russian IP address and then harvested donors' details, sources said. The Kremlin - and Vladimir Putin - have denied any involvement

The IP address of the dodgy website appeared to be similar to one used by Russian government-linked hackers suspected in the breach of the Democratic National Committee, a separate body that sets strategy and raises money for the Democratic Party nationwide.

The DNC and DCCC share the same office building in Washington, DC.

The DCCC and donation processing company ActBlue did not comment.

The leaked emails last week revealed that DNC Chairwoman Debbie Wasserman Schultz and other senior party officials were biased against Hillary Clinton's main rival for the nomination, Bernie Sanders.

Voicemails that were also contained among the more than 19,000 leaked files showed that donors were also lobbying party officials to drop Sanders and ban him from speaking at this week's convention.

Cyber security experts and U.S. officials have said there was evidence that Russia engineered the original DNC hack to release sensitive party emails in order to influence the presidential election.

Leaked emails last week revealed that DNC Chairwoman Debbie Wasserman Schultz and other senior party officials were biased against Hillary Clinton's main rival for the nomination, Bernie Sanders

Sources claimed last night that Russians may have deliberately left digital fingerprints on the leaked files in an attempt to show Washington, DC, that they are a power to be reckoned with.

'Either these guys were incredibly sloppy, in which case it's not clear that they could have gotten as far as they did without being detected, or they wanted us to know they were Russian,' said the official.

Donald Trump raised suspicions - and eyebrows - when he appeared to suggest the Russians were involved, but he later claimed he was joking.

'I have concerns that an agency of foreign intelligence is hacking and interfering with a U.S. election,' Clinton campaign chairman John Podesta said, adding that he was unaware of the DCCC hack.

An FBI spokesman said the agency were taking the matter 'very seriously' and would look to apprehend 'those who pose a threat in cyberspace'.

Director of National Intelligence James Clapper said on Thursday the U.S. intelligence community was not yet sure who was behind the hack, but added: 'There's just a few usual suspects out there.'