LONDON — Crossing a significant threshold in its battle with jihadist groups, Britain announced on Monday that it had conducted an armed drone strike for the first time inside Syria, killing three suspected members of the Islamic State, including two British citizens.

In a statement to Parliament, Prime Minister David Cameron said that a British man, Reyaad Khan, had been identified as a terrorist threat and was killed on Aug. 21 in an operation in which two associates, including another Briton, identified as Ruhul Amin, also died.

Mr. Cameron’s announcement came as France said it would start sending reconnaissance flights over Syria and was considering airstrikes against the Islamic State in Syrian territory.

The British government’s decision to order a lethal drone strike against a terrorism suspect in another country — an action long practiced by the United States — is a significant step, raising questions about the scope of British operations in the Middle East.