A formerly Christian American man has been killed while fighting for ISIS, Washington has confirmed.

Douglas McAuthur McCain, 33, was killed in a fight between two Syrian opposition groups last weekend.

Having grown up in Minnesota, McCain converted to Islam a decade ago according to his Twitter account, where he wrote that this decision was "the best thing that ever happen to me" [sic].

His uncle told reporters that prior to his conversion, McCain was "a good person, loved his family, loved his mother, loved his faith."

According to news reports, he travelled to Syria earlier this year to join the Islamic State (IS). Following his death in a clash with the opposing Syrian Free Army, his body was found along with his American passport and $800 in cash.

McCain's family was informed on Monday of his death.

US National Security Council spokeswoman Caitlin Hayden subsequently released a statement confirming: "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."

An unnamed senior government oficial told NBC news that dozens of US jihadists like McCain have travelled to Iraq and Syria in recent months.

"The threat we are most concerned about to the homeland is that of fighters like this returning to the US and committing acts of terrorism," he warned.

State Department spokeswoman Jen Psaki said on Tuesday that controlling would-be jihadist's movements is a top priority for the Administration.

"The issue of foreign fighters and the concern of individuals with Western passports or passports that would enable them to travel into countries where they can do harm is certainly at the top of our agenda and the top of the agenda of many countries," she confirmed.

Psaki also added that Barack Obama will be involved in a UN General Assembly meeting in September, focusing on the threat of foreign terrorist fighters.