(USA TODAY) -- The Obama administration is investigating reports from Syria that a second American was killed over the weekend while fighting alongside Islamist State extremists.
The news comes a day after U.S. officials confirmed that 33-year-old Douglas McCain, who had lived in Southern California and Minnesota, died in a battle between the Islamic State and other Syrian opposition groups.
The other American was not identified.
The coalition, called the Nahrawan of Syria, said two Americans died north and east of Aleppo on Sunday, CNN reported.
NBC News cited a Free Syrian Army source as saying another American jihadist fighting with the Islamic State also was killed. NBC also quoted an unnamed law enforcement source as saying the report of the second American's death "appears to be true" but offered no details.
The National Security Council said in a statement Wednesday evening that it was "aware of media reporting and social media activity indicating that a 2nd American citizen associated with" the Islamic State had been killed in Syria. "At this point, we are not in a position to confirm those reports."
The Islamic State has not said whether any of its slain fighters have been Americans.
Also Wednesday, a U.N. commission accused the Islamic State of crimes against humanity with attacks on civilians. The U.N. report emerged as photos posted by the IS show the extremists' bloody takeover of the Tabqa air base on Sunday, including images of seven kneeling men being executed.
The panel also found that President Bashar al-Assad has likely used chlorine gas to attack civilians in its battle against the Islamic State and other opposition groups.