BEIRUT (AP) — The Latest on the conflict in Syria (all times local):

3 p.m.

A U.S.-backed Syrian Kurdish coalition says the U.S. has landed infantry behind Islamic State lines to spearhead an assault on the town of Tabqa.

The Syrian Democratic Forces say the U.S. has airlifted several Marines and SDF fighters by helicopter into Islamic State-held territory, capturing four villages, and cutting the main artery running between the extremists' de facto capital, Raqqa, and the western countryside.

It was not immediately possible to confirm the deployment with U.S. commanders.

The group said in a statement on social media Wednesday that the operation was in preparation for an assault on Tabqa, an IS redoubt 45 kilometers (28 miles) west of Raqqa.

The activist-run group Raqqa is Being Slaughtered Silently said 500 U.S. and Syrian Kurdish forces were deployed in the operation Wednesday morning. The group relies on local contacts to smuggle information out of IS territory.

11:15 a.m.

Syrian activists say dozens of civilians are dead or still missing after an airstrike on a shelter for the displaced in an Islamic State-held village in northern Syria.

The activist-run Raqqa is Being Slaughtered Silently says a school sheltering some 50 families in Mansoura was leveled by airstrikes on Tuesday morning. Mansoura is 26 kilometers (16 miles) west of Raqqa, the de facto capital to the extremists' so-called caliphate.

The activist group said Wednesday morning the families were still unaccounted for.

The Britain-based Syrian Observatory for Human Rights said Wednesday that 33 bodies had been pulled from the rubble.

The two organizations rely on local contacts to smuggle news out of Islamic State-held territory.