Islamic State fighters tightened their grip on the historic Syrian city of Palmyra on Thursday and overran Iraqi government defenses east of Ramadi, the provincial capital that they seized five days earlier.
The twin successes not only pile pressure on Damascus and Baghdad but throw doubt on a U.S. strategy of relying almost exclusively on air strikes to support the fight against Islamic State.
U.S. and coalition forces had conducted 18 air strikes against Islamic State targets in Syria and Iraq since Wednesday, the U.S. military said.
The British-based Syrian Observatory for Human Rights said the al Qaeda offshoot now controlled more than half of all Syrian territory after more than four years of conflict that grew out of an uprising against President Bashar al-Assad.
The monitoring group added that Islamic State had seized the last border crossing between Syria and Iraq controlled by the Damascus government. The crossing is in Syria's Homs province, where Palmyra is located.