US forces killed two "Islamic State of Iraq and Syria" (ISIS) "officials" during a "successful helicopter raid" in eastern Syria in the pre-dawn hours of Sunday (December 11), the US military said.
"Extensive planning went into this unilateral operation to ensure its success," US Central Command (CENTCOM) said in a statement.
It identified one of those killed in the raid, carried out at 2.57am local time, as "Anas", an ISIS "Syria province official" who was involved in "plotting and facilitation operations in eastern Syria".
The Syrian Observatory for Human Rights said it was the "most prominent" anti-ISIS operation for at least three weeks.
US forces carried out two raids in October that killed three ISIS leaders and wounded an associate.
Observatory chief Rami Abdel Rahman said the anti-terrorism unit of the Syrian Democratic Forces (SDF) also took part in Sunday's operation, identifying the village of al-Zor in eastern Deir Ezzor province as the target area.
Observatory activists reported that the two people killed were local commanders of ISIS -- an Iraqi and a Syrian -- who were responsible for assassinations in al-Zor and al-Basira in rural Deir Ezzor.
US commitment to ISIS defeat
"The United States remains committed to countering the global threat from ISIS in partnership with local forces," CENTCOM said in a statement, noting that "ISIS continues to pursue an aggressive operational agenda".
"ISIS continues to represent a threat to the security and stability of the region," said CENTCOM spokesman Joe Buccino. "This operation reaffirms CENTCOM's steadfast commitment to ensuring the group's enduring defeat."
"The death of these ISIS officials will disrupt the terrorist organisation's ability to further plot and carry out destabilising attacks in the Middle East," he said.
Separately, an Asayesh (Kurdish internal security forces) patrol on Sunday stormed a house in al-Raqa to arrest a person from al-Jarka village who was accused of "belonging to ISIS", the Observatory said.
He refused to surrender and detonated a grenade, it said, blowing himself up and seriously injuring three members the Asayesh in the process.
On November 30, ISIS announced that its leader Abu Hasan al-Hashimi al-Qurashi had been "killed in battle".
But local sources said the top ISIS leader had in fact blown himself up under siege -- following in the footsteps of his short-lived predecessors.