'US Isis fighter' Douglas McAuthur McCain dies 'fighting alongside Islamic State militants' in Syria

McCain was reportedly found dead by Free Syrian Army fighters

A US official said Douglas McAuthur McCain, a US citizen, is believed to have been killed in Syria and was there to fight alongside a terrorist group, most likely the Islamic State group (AP)

A 33-year-old US citizen who is believed to have travelled to Syria to fight alongside Islamic State (Isis) militants has died in battle over the weekend.

A relative of Douglas McAuthur McCain, who grew-up in Minnesota, said the State Department had confirmed his death to his family on Monday.

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

"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.”

Members of the Free Syrian Army said McCain was killed during fighting at the weekend, NBC News reports. The broadcaster claims it has seen photographs of McCain’s passport and of body.

Officials do not yet have his body and are still trying to verify information surrounding his death, an official told AP.

It is unclear when McCain, who had most recently lived in San Diego, California, travelled to Syria. His uncle Ken McCain said his nephew converted from Christianity to Islam “several years ago”.

He told CNN: "We are devastated and we are just as surprised as the country is."

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

"I didn't know that he was in support of Isis”, she told the newspaper. "I didn't think he would be."

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

Mr Comey said the terrorism threat of today "has spread, metastasized. ... The traveller problem makes it even more difficult, because the people going to Syria are not from any particular demographic. They're not from any particular part of the United States."

The US began surveillance flights over Syria on Tuesday to monitor Isis militants advancing across the country. The flights, comprising both manned aircraft and drones, were green-lighted by President Obama over the weekend, officials told AP.

The move comes amid news that a female humanitarian worker has become the third identified American who has been kidnapped and held hostage by Isis. The woman is understood to have been working in Syria when she was taken captive last year.

Just over a week ago Isis released video footage showing the beheading of American photo journalist James Foley, alongside another missing American journalist Steven Sotloff. The group warned Mr Sotloff would be killed next if the US continued air strikes in Iraq.