BEIRUT (AP) — Syrian opposition activists are reporting clashes in the southern city of Daraa between government forces and insurgents, saying that government aircraft have dropped barrel bombs on the city.

The Britain-based Syrian Observatory for Human Rights said the fighting broke out when rebels launched an offensive on a government-held area in central Daraa on Monday. It said that by Tuesday, 16 pro-government fighters, including an army colonel, had been killed.

Daraa-based activist Ahmad al-Masalmeh and the Observatory said warplanes carried out raids on Daraa while helicopter gunships dropped at least eight barrels loaded with explosives onto the city.

White House spokesman Sean Spicer said Monday that the use of barrel bombs, which government forces have repeatedly employed throughout the six-year-old conflict, might bring a U.S. response.