Facebook CEO Mark Zuckerberg has told US senators his company is in a constant battle with Russian operators seeking to exploit the social network.

"This is an arms race. They're going to keep getting better," he said.

Mr Zuckerberg was answering questions in the wake of the Cambridge Analytica data collection scandal.

He also revealed Robert Mueller, the special counsel investigating alleged Russian meddling in the 2016 election, had interviewed Facebook staff.

But he added: "Our work with the special counsel is confidential and I want to make sure that in an open session I'm not revealing something that's confidential."

In February, Mr Mueller's office charged 13 Russians with interference in the 2016 election, along with three Russian companies.

One was the Internet Research Agency, sometimes referred to as a "Russian troll farm", which the indictment said had a "strategic goal to sow discord in the US political system".

Mr Zuckerberg said the company was now developing new tools to identify fake accounts.

"There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well. We need to invest in getting better at this too."

The Facebook chief fended off questions from senators about how the social network might be regulated more closely.

Senator John Kennedy warned him: "I don't want to have to vote to regulate Facebook. But by God, I will. That depends on you... Your user agreement sucks."

When pressed, the 33-year-old billionaire tech titan said he would welcome regulation, if it was the "right regulation," though he avoided specifics.

He was appearing in front of a joint session of several US senate committees, after it was revealed in recent weeks that about 87 million people had their profile information accessed by marketing firm Cambridge Analytica.

During the hearing, Mr Zuckerberg also said:

• "It's clear now that we didn't do enough to prevent these tools from being used for harm"
• "In retrospect it was clearly a mistake" to believe Cambridge Analytica deleted data, without further examination
• He does not "feel like" Facebook has a monopoly
• That there would always be a free version of Facebook, leaving open the possibility of a paid, ad-free version of the social network
• Dealing with hate speech automatically has "a higher error rate than I am happy with"
• He was personally concerned about the possibility of political bias at the company

By the first break in proceedings, Facebook's share price had risen by almost 5%, as markets reacted favorably to Mr Zuckerberg's performance, increasing his net worth by an estimated $3bn.

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