Mark Zuckerberg made the disclosure during his second day of being questioned in Washington.

He also revealed that his firm was exploring whether it needed to take action against the University of Cambridge.

The institution is where the researcher who had collected and sold personal data to Cambridge Analytica was based.

"What we found now is that there's a whole programme associated with Cambridge University where... there were a number of other researchers building similar apps," Mr Zuckerberg said.

"So, we do need to understand whether there was something bad going on at Cambridge University overall that will require a stronger reaction from us."

A spokeswoman for the university said it was preparing a response.

Mr Zuckerberg had earlier apologised for having failed to check in 2015 that Cambridge Analytica had deleted information gathered about millions of Facebook users, after learning that the political consultancy had acquired the records.

"We have a responsibility to make sure what happened with [app developer] Kogan and Cambridge Analytica doesn't happen again," he said.

The 33-year-old added that Facebook's audit of other apps would take many months to complete. But he said that the firm had seen no evidence as yet that Russia or China had attempted to scrape people's information.

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• Facebook: Data contained private messages

Mr Zuckerberg's appearance before the House Committee on Energy and Commerce follows a five hour session in front of two Senate committees yesterday.

During a testy early exchange, he declined to give a commitment to change all users' default privacy settings to collect the minimum amount of personal information.

"This is a complex issue," Mr Zuckerberg said.

"That's disappointing to me," responded Democratic congressman Frank Pallone.

Elsewhere in the hearing, Mr Zuckerberg acknowledged that he believed it was "inevitable" that the internet would need new regulations.

"My position is not that there should be no regulation, but I also think you have to be careful about what regulation you put in place," he said.

The Facebook chief also faced fresh accusations from Republican congressman Steve Scalise that the News Feed's algorithm was discriminating against conservative news and content in favour of liberal posts.

"There is absolutely no directive in any of the changes that we make to have a bias," responded Mr Zuckerberg.

"To the contrary our goal is to be a platform for all ideas."

Other developments over the past day include:

• The European Commissioner for consumers and justice has told the Guardian she may propose new regulations to tackle a "loss of trust" in Facebook, and would raise the matter with the tech firm's chief operating officer Sheryl Sandberg later this week
• Cambridge Analytica has sent letters to publishers including the BBC warning that it will treat any misleading or inaccurate reports about itself with the "utmost seriousness"
• The UK Culture Secretary Matt Hancock has met with Facebook officials in London