The United States was investigating Wednesday news reports that a second American was killed recently fighting for an Islamist extremist group in Syria.
"We have seen those reports. We're looking into it but we don't have any independent confirmation at this point in time," State Department spokeswoman Jen Psaki told reporters.
US officials on Tuesday confirmed that American Douglas McCain, 33, was killed in Syria fighting for the Islamic State, which aims to establish a Muslim caliphate in the Middle East.
The American jihadist traveled to war-torn Syria to join the group, which has marauded across swathes of Iraq and Syria in recent months and beheaded US journalist James Foley more than a week ago.
McCain was reportedly killed in fighting against the Al-Nusra Front, an Al-Qaeda-linked group blacklisted by the United States.
NBC News reported it was told by a source from the Free Syrian Army -- western-backed rebels fighting the regime of Syria's President Bashar al-Assad -- that the second American was killed in the same battle.
Fears have swelled in recent weeks about the flood of foreigners into Syria, with no end in sight to the three-year war that has already left 162,000 people dead.
Some 12,000 foreign jihadists from 50 countries have gone to fight in Syria since the conflict began, the US State Department said last week.
Psaki reiterated Wednesday that "up to 100" Americans are believed to be among them.
In May, US officials confirmed that a US citizen, believed to be Moner Mohammad Abu-Salha and fighting for a hardline Islamist group, carried out a deadly suicide bombing against Syrian regime forces, in the first such case in the war.
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