Two senior members of ISIS were killed in Syria's Hasakah province in an air strike on Monday, a monitoring group said.
The Syrian Observatory for Human Rights, a UK-based group which gathers information from activists on the ground, identified one of the men as Abu Osama al Iraqi and the other commander as Amer al Rafdan.
The group did not specify whether the men were killed by US-led coalition or the Syrian army.
ISIS has not made any comment about the deaths.
Hasakah province in northern Syria has seen an upsurge of fighting in the last few weeks, mostly between ISIS and the Kurdish Democratic Union Party (PYD) militants, forcing thousands of people to flee their homes.
Hasakah is close to the border with Turkey. The PYD militants are reported to be dislocating Turkmens, who returned to their homes after seeking refuge in Turkey, in a bid to form a Kurdish majority in northern Syria.
The PYD is considered by Turkey to be the Syrian affiliate of the outlawed PKK which is recognised as a terrorist group by Turkey, the US and the EU.