Activists say U.S. airstrikes on a town in northern Syria killed at least 73 civilians on Tuesday. The monitoring group Air Wars says it is the deadliest U.S. airstrike for civilians to date since the U.S. began strikes in Syria in 2014. A U.S.-supported militia called the Syrian Democratic Forces began an assault on the town of Manbij at the end of May. Since then, groups that monitor the war say at least 190 civilians have been killed in U.S. airstrikes in the town. The U.S. military has said it is investigating the incident. Until earlier this year, the Pentagon claimed no civilians had been killed in its air campaign against ISIS, while at the same time claiming more than 20,000 ISIS fighters had been killed.