BEIRUT -- A U.S.-led special forces raid on Islamic State of Iraq and Syria (ISIS) militants in eastern Syria has reportedly left two dozen of the jihadist fighters dead.

The U.S. military command in Baghdad confirmed American forces’ involvement in the raid, which took place on Sunday, without elaborating.

Omar Abou Leila, a Syrian activist who runs the Deir Ezzor 24 group, said four helicopters landed in the desert between the ISIS-held cities of Deir el-Zour and Raqqa on Sunday. Commandos set up checkpoints and intercepted a vehicle carrying several ISIS militants, killing all of them and flying off with the bodies, he said.

The Britain-based Syrian Observatory for Human Rights, another activist-run group, said 25 militants were killed in the ambush.

Almost all of Deir el-Zour province, which sits along Syria’s border with Iraq, remains under ISIS control, and the U.S. has carried out airstrikes targeting the group there for more than a year.

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“It’s an operation that apparently targeted an important figure,” Abou Leila told The Associated Press from Germany, where he is based. Deir Ezzor 24 is one of several locally staffed underground groups reporting from ISIS-held territory.

The U.S. military spokesman in Baghdad who confirmed the raid did not say which senior ISIS members might have been targeted, however, and there was no indication that the group’s leader, Abu Bakr al-Baghdadi, was at either of the two sites hit by American forces.

Syrian President Bashar Assad said in remarks published Monday, meanwhile, that he was prepared “to negotiate everything” at planned talks later this month in Kazakhstan, seeking to cast himself as a peacemaker after his forces’ recapture of Aleppo last month.

However, the upcoming talks, brokered by Ankara and Moscow, are still in doubt as Syrian opposition groups have yet to confirm their participation. The main opposition umbrella group said less than two weeks ago that it was suspending all discussions about the pending talks because Russian and Syrian forces were, according to the rebels, violating the tenuous cease-fire in the country.

Speaking to French reporters at his Damascus palace, Assad defended his troops’ deadly bombardment of eastern Aleppo, saying the alternative would have been to leave the city’s civilians to the mercy of “terrorists” -- a term the government uses for all rebels.

Assad questioned the credibility of Syrian opposition groups backed by the West and Saudi Arabia, which make up the bulk of the armed and political opposition to his rule.

“There’s no limit to negotiations,” Assad said, in remarks carried by Syrian state media. “But who is going to be there from the other side, we don’t know yet. ... The viability of the conference depends on that.”

Past Syrian peace talks have run aground on the question of Assad’s future and whether he is to continue as president, with the opposition insisting his departure is a precondition for any reforms.

Assad said the matter could only be resolved through a constitutional referendum.

“If they want to discuss this point, they need to discuss the constitution. You need a referendum for every (constitutional amendment). This is one of the points that could be discussed in the meeting” in Kazakhstan, he said.

The talks are scheduled to begin in the Kazakh capital of Astana on Jan. 23. They follow a lengthy rapprochement between Russia, a key backer of Assad, and Turkey, a main sponsor of the opposition, that culminated in the cease-fire agreement that came into force on Dec. 30, but which has already started to erode.

Russian officials have suggested the U.S. could be invited to the talks at a later date.

The Obama administration has been at odds with Russia over how to resolve Syria’s conflict. Incoming President Donald Trump has indicated he might distance the U.S. from Syria’s rebels, bringing Washington in closer alignment with Moscow.

Trump wrote on Twitter Saturday that he would strive to build good relations with Russia, and “perhaps, work together to solve many of the great and pressing problems and issues of the WORLD!”

Asked about the comments, Assad said warmer relations between Washington and Moscow “reflects positively on the Syrian conflict.”

As for Aleppo, Assad said the government forces were forced “to liberate” the city.

“There is a price, sometimes, but at the end the people are liberated from the terrorists,” he said.

Once Syria’s largest city and industrial hub, Aleppo has been devastated by nearly six years of war. Rebels took control of its eastern districts in 2012, before surrendering it to government authority last month.

The U.N. said the government’s relentless military campaign, which displaced tens of thousands of civilians, could have violated the laws of war.