U.S.-led warplanes killed a Daesh leader on Sunday in Syria responsible for executing hostages including an American, Washington's envoy to the coalition fighting the terrorist group said.

"Earlier today, coalition air forces conducted precision strikes against a number of [Daesh] leaders in southeast Syria. Those targeted included Abu al-Umarayn," U.S. special envoy Brett McGurk said in a Tweet late on Sunday.

Abu al-Umarayn was responsible for killing several prisoners including the U.S. aid worker Peter Kassig, who was captured by the group in Syria and beheaded in 2014, McGurk said. The strike in the Badia desert in eastern Syria killed "several other [Daesh] members," said Colonel Sean Ryan, spokesman for the U.S.-led coalition against Daesh. Abu al-Umarayn "had given indications of posing an imminent threat to coalition forces," Ryan said.

Late on Sunday, Syrian media reported that the United States had fired missiles at Syrian regime positions in the desert in eastern Syria. Ryan said the coalition did not fire at any Syrian forces.

Daesh has lost almost all the territory in Iraq and Syria where it had once declared a caliphate, including its two main cities of Mosul and Raqqa last year. The U.S.-led coalition is supporting the Syrian Democratic Forces (SDF), a group that mainly consists of People's Protection Units (YPG) terrorists, assaulting the last Daesh territory in eastern Syria near the Iraqi border. The coalition said in a statement it was supporting the SDF ground operation with air and artillery strikes. Turkey recognizes the PKK and YPG as organically linked terrorist groups. The U.S., however, while listing the PKK as a terrorist group, has supported the YPG militarily, under the pretext of fighting Daesh.