A US man who is thought to have been killed in Syria had gone there to fight alongside an extremist militant group, most likely the Islamic State, a US official said.

Investigators were aware that Douglas McCain was in the country to support the militant group, but they did not yet have his body and were trying to verify information about his presumed death, according to the official, who was not authorized to discuss by name an ongoing investigation and spoke only on condition of anonymity.

A relative, Kenneth McCain, said on Tuesday that the US State Department had called to tell his family that Douglas McCain had been killed in Syria. "We do not know if he was fighting anyone," McCain said.

US officials, concerned over what they say is a growing threat posed by the extremist Islamic State group, say surveillance flights have begun over Syria on the orders of President Barack Obama. The move could pave the way for airstrikes against the radicals, who control a large part of eastern Syria and who crossed into Iraq earlier this year. The militant group also killed US journalist James Foley and is holding a US woman hostage.

It was unclear when Douglas McCain, who had most recently lived in San Diego, traveled to Syria. He grew up outside Minneapolis in the town of New Hope.

A cousin, Kenyata McCain, told The Minneapolis Star Tribune that she had spoken to him as recently as Friday, and "he was telling all of us he was in Turkey".

"I know that he had strong Muslim beliefs," she said. "But I didn't know that he was in support of ISIS. I didn't think he would be."

At an apartment complex in New Hope, Shelly Chase remembered McCain as a friendly boy who welcomed her 9-year-old son, Isaac, when the Chase family moved in some two decades ago. Even though McCain was a few years older, the boys lifted weights, hit punching bags and played basketball together.

Both Chase and her son, now 28, fought back tears as they talked about McCain.

"I'm holding in the tears, I really am, because this is hard. He was a good kid," Chase said. "Someone must have persuaded him."

US law enforcement and intelligence officials have expressed concern about the influence of hard-line jihadists, who are among the rebels seeking to overthrow Syrian President Bashar al-Assad. Officials say sympathizers from the US or Europe looking to join the cause could become radicalized and import those influences and terrorist skills when they return home.

FBI Director James Comey said in June that roughly 100 people had left the United States to join the conflict in Syria.