LONDON — Prime Minister Theresa May is urging internet companies to block the spread of extremist material, calling on social media giants like Facebook, Twitter and Google to develop technologies that will prevent content from being posted in the first place.

Britain’s leader will focus on the fight against extremist content during a meeting with internet companies Wednesday at the U.N. General Assembly in New York.

May says that while social media platforms have made progress in fighting extremist propaganda, they need to ensure content is removed in less than two hours.

She will say that “industry needs to go further and faster in automating the detection and removal of terrorist content” because extremists “are placing a greater emphasis on disseminating content at speed in order to stay ahead” of surveillance.