Washington: Facebook chief executive officer Mark Zuckerberg defended on Thursday his decision to encrypt the company's messaging services, despite concerns about its impact on child exploitation and other criminal activity.

Facebook is in the sights of regulators.

The United States, Britain and Australia signed an open letter earlier in the day calling for Facebook to suspend its encryption plan, saying it would hinder the fight against child abuse and terrorism.

Zuckerberg, speaking in a livestream of the company's weekly internal Q&A session, said he had been aware of child exploitation risks before announcing his encryption plan and acknowledged that it would reduce tools to fight the problem.

"When we were deciding whether to go to end-to-end encryption across the different apps, this was one of the things that just weighed the most heavily on me," he said.