U.S. officials said Tuesday that an American convert to Islam who recently traveled to Syria apparently has been killed in fighting. The circumstances surrounding the death of Douglas McCain, 33, remain unclear.

McCain was reportedly killed as rival militant factions fought near Aleppo, but officials could not say whether he was fighting for the Islamic State or another group.

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

NBC News first reported McCain’s death. In an interview with CNN, McCain’s uncle said his nephew had traveled several months ago to Turkey, a major transit point for foreign fighters hoping to cross into Syria. A U.S. official said law enforcement officials were aware that McCain had traveled to Syria.

McCain’s Twitter account with the handle “Iamthetooth” features a series of pro-Islamic State tweets, including a recent retweet reading: “It takes a warrior to understand a warrior. Pray for ISIS.” ISIS is an acronym for the militant group.

Rebel fighters walk in front of damaged buildings in Karam al-Jabal neighborhood of Aleppo, Syria, on Tuesday. (Zein Al-Rifai/AFP/Getty Images)

About 100 Americans have traveled to Syria since the conflict began almost four years ago, and at least two have died there, U.S. officials said.

Moner Mohammad Abusalha, a 22-year-old Florida man fighting for Jabhat al-Nusra, committed suicide in May when he drove a dump truck laden with explosives into a Syrian army position, while Nicole Lynn Mansfield, 33, of Michigan was killed a year earlier fighting with Syrian rebels.

Foreign fighters in Syria are a major concern for the United States and European nations that fear they may return home battle-hardened and determined to launch terrorist attacks in the West. FBI officials say they are tracking a dozen or so former fighters who have returned to the United States.

NBC reported that McCain attended college in San Diego but it is unclear whether he graduated.

His sister, Delecia McCain, posted a message on Facebook earlier Tuesday expressing her shock and disbelief about her brother’s death.

“I really don’t understand why and how and I have no words, I never thought this will be the way we say goodbye,” she wrote.

McCain’s last tweet was on Aug. 16. It was a retweet of a picture of two men standing in a room and had a quote across the bottom of the frame that read, “Those who are slaves to their own desires will never understand the reason why we give up comfort for a life of struggle.”

Julie Tate contributed to this report.