WASHINGTON (Reuters) - A U.S. Navy SEAL killed by Islamic State militants in northern Iraq on Tuesday was part of a “quick reaction force” called in after a firefight broke out involving American advisers, a U.S. military spokesman said on Wednesday.

Islamic State militants breached the Kurdish forward lines at about 07:30 a.m. local time (12.30 a.m. ET) while an American team of advisers was at the village of Tel Asqof, approximately 3.5 km (2.2 miles) away, the spokesman said.

The American advise and assist team called in the quick reaction force about 20 minutes later after becoming involved in a firefight with Islamic State forces, he said.

Petty Officer First Class Charles Keating, a member of the quick reaction force which was comprised of special operations forces, was struck by “direct fire” at approximately 9:30 a.m. local time (0630 GMT), Army Colonel Steve Warren told a Pentagon briefing.

Keating was evacuated within an hour, but died of his wounds, Warren said.

The elite serviceman was the third American to be killed in direct combat since a U.S.-led coalition launched a campaign in 2014 to “degrade and destroy” Islamic State and is a measure of its deepening involvement in the conflict.

Coalition aircraft carried out air strikes after the attack, killing more than 50 Islamic State militants, Warren said, adding that the attack included at least 145 Islamic State militants and was one of the largest recent attacks by the group.

A quick reaction force was formed on a mission by mission basis depending on the threat level, distance of the threat and other factors, Warren added.

“I don’t want you to think that there is some sort standing (quick reaction force) out there in the sky somewhere that can respond to anything that happens across the entire battle field,” he said.