Two senior Islamic State leaders were killed in an air strike in northeastern Syria on Monday, the Syrian Observatory for Human Rights reported.
The Observatory, which gathers its information from an activist network on the ground, identified the leaders as an Iraqi, Abu Osama al-Iraqi, and a Syrian named Amer al-Rafdan.
Rami Abdulrahman, who runs the Observatory, said it was not clear whether the air strike in Hasaka province was carried out by the Syrian army or a U.S.-led coalition that is mounting a separate aerial campaign against Islamic State in Syria.
The ultra-hardline Islamic State is fighting in Hasaka province against both Syrian government forces and militia and Syrian Kurdish fighters who have received air support from the U.S.-led coalition. Hasaka province borders Turkey to the north and Iraq to the east.
U.S. President Barack Obama said last week the coalition was intensifying its campaign in northern Syria. The Kurdish YPG militia have made a series of advances against Islamic State in the north, including the capture of the town of Tel Abyad at the Turkish border.
Abdulrahman said al-Iraqi had the title of "governor" of the province Islamic State had declared in northeastern Syria. Rafdan had previously served as Islamic State\’s governor of Deir al-Zor province, he said.
Islamic State last year declared a cross-border "caliphate" claiming to rule over all Muslims. The U.S.-led alliance has ruled out the idea of cooperation with Syrian President Bashar al-Assad, seeing him as part of the problem.