The Democratic National Committee last night said it had thwarted an attempt to hack into a database containing information on tens of millions of voters across the US.

A sophisticated "spear phishing" operation used a fake login page in a bid to secure usernames and passwords, and gain access to the database.

It was not immediately clear who was responsible for the attack.

In 2016 Russian hackers gained access to DNC servers and released embarrassing to Hillary Clinton's presidential campaign.

A party official said, on this occasion, no information was compromised, and the FBI had been called in.

The FBI did not immediately respond to a request for comment.

Bob Lord, the DNC's chief security officer, briefed leaders of state Democrat parties on the attempt at a gathering in Chicago on Wednesday.

The DNC distributes the voter files to state parties for Democrat candidates to use during campaigns.

Mr Lord said: "This attempt is further proof that there are constant threats as we head into midterm elections and we must remain vigilant in order to prevent future attacks.

"We can't do this alone. We need the administration to take more aggressive steps to protect our voting systems."

In the latest hacking attempt a San Francisco-based security company detected the fake website page, which had been designed to harvest the passwords of users. It was taken down within hours.