Updated

Syrian opposition activists say Islamic State militants have launched a new offensive on government-held areas in the contested eastern city of Deir el-Zour.

They report intense fighting Saturday between Syrian troops and IS members in the city and in the vicinity of a nearby military airport controlled by government forces.

The extremists, who control most of Deir el-Zour province, have kept the provincial capital under siege since 2014. Government forces have withstood the encirclement thanks to air-dropped humanitarian assistance and weapons and ammunition flown into the airport.

The Syrian Observatory for Human Rights says Saturday's offensive was the most intense attack in a year. Others reported loud explosions that shook the city and said Syrian warplanes were taking part in the battles.