WASHINGTON/BEIRUT (Reuters) - The U.S. military is investigating reports that an air strike near Manbij, Syria, on Thursday caused civilian casualties, U.S. Central Command said in a statement.

The U.S.-led coalition fighting Islamic State militants conducted air strikes in the area in the last 24 hours, the statement said.

The Syrian Observatory for Human Rights said overnight on Thursday at least 28 people, including seven children, died in air strikes on the village of al Ghandour in the countryside north of Manbij city. The international coalition was believed to have conducted the air strikes, the group said.

A U.S.-backed Kurdish and Arab alliance, known as the Syria Democratic Forces (SDF), has been fighting Islamic State in northern Syria with the support of air strikes by a U.S.-led coalition.

The SDF fighters, who quickly took control of the territory surrounding Manbij last month, have in recent weeks taken western parts of the city in slower advances.

The anti-Islamic State U.S.-led coalition said last week it was investigating another report of civilian casualties in an air strike near Manbij.

(Reporting by Eric Beech in Washington and LIsa Barrington in Beirut; Editing by Eric Walsh, Larry King)