Facebook Chief Executive Mark Zuckerberg said his company would step up efforts to block hate messages in Myanmar as he faced questioning by the U.S. Congress about electoral interference and hate speech on the platform.

Facebook has been accused by human rights advocates of not doing enough to weed out hate messages on its social-media network in Myanmar, where it is a dominant communications system.

“What’s happening in Myanmar is a terrible tragedy and we need to do more,” Zuckerberg said during a five-hour joint hearing of the Senate Commerce Committee and Senate Judiciary Committee.

More than 650,000 Rohingya Muslims have fled Myanmar’s Rakhine state into Bangladesh since insurgent attacks sparked a security crackdown last August.

United Nations officials investigating a possible genocide in Myanmar said last month that Facebook had been a source of anti-Rohingya propaganda.

Marzuki Darusman, chairman of the UN Independent International Fact-Finding Mission on Myanmar, said in March that social media had played a “determining role” in Myanmar.

“It has … substantively contributed to the level of acrimony and dissension and conflict … within the public. Hate speech is certainly of course a part of that. As far as the Myanmar situation is concerned, social media is Facebook, and Facebook is social media,” he said.

Zuckerberg said Facebook was hiring dozens more Burmese-language speakers to remove threatening content.

“It’s hard to do it without people who speak the local language, and we need to ramp up our effort there dramatically,” he said, adding that Facebook was also asking civil society groups to help it identify figures who should be banned from the network.

He said a Facebook team would also make undisclosed product changes in Myanmar and other countries where ethnic violence was a problem.