By Dan Whitcomb

LOS ANGELES (Reuters) - An American man suspected of fighting alongside Islamic State militants who have seized large areas of Iraq and Syria to the alarm of the Baghdad government and its allies in the West has been killed in Syria, a U.S. official said on Tuesday.

"We were aware of U.S. Citizen Douglas McAuthur McCain's presence in Syria and can confirm his death," U.S. National Security Council spokeswoman Caitlin Hayden said in a statement.

"We continue to use every tool we possess to disrupt and dissuade individuals from traveling abroad for violent jihad and to track and engage those who return," Hayden said.

A national security official who asked not to be named told Reuters that the FBI was investigating McCain's death, and a State Department spokeswoman said officials had been in contact with his family and were providing "all consular assistance."

Family members told the Minneapolis Star Tribune that McCain's mother had been called by a State Department official to report that he had been killed in Syria over the weekend.

The newspaper said the family had been concerned with McCain's expressions of support of Islamic State militants, and the man's uncle, Ken McCain, told CNN his nephew had converted to Islam from Christianity several years ago and traveled to the region via Turkey to fight as a jihadi.

That could place him among the dozens of U.S. citizens that Attorney General Eric Holder said in July were included in a cohort of roughly 7,000 foreign fighters that U.S. intelligence agencies estimate to be operating in Syria, out of roughly 23,000 violent extremists.

Holder said federal prosecutors had opened fewer than 100 investigations into American citizens who may have traveled to Syria or Iraq to fight.

NBC News reported that McCain was born in Illinois and moved with his family to the Twin Cities area, where he graduated from high school in the Minneapolis suburb of New Hope in 1999. He later moved to the San Diego area, where he attended community college.

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His death comes after a 22-year-old man from Florida carried out a suicide bombing in Syria's Idlib province in May. Later, a Denver woman was arrested in July on suspicion of trying to fly to Syria to support insurgents, and two men in Texas were taken into custody on similar charges in June.

One of the Texas men was charged with "attempting to provide material support to terrorists," a law that Holder urged other countries to copy as vital to counter terrorism efforts.

(Reporting by Mark Hosenball and Warren Strobel in Washington, Marty Graham in San Diego, David Bailey in Minneapolis and Dan Whitcomb in Los Angeles; Writing by Dan Whitcomb; Editing by Cynthia Johnston, Eric Walsh and Sandra Maler)