Syrian opposition activists say Islamic State militants have attacked U.S.-backed fighters near an oil field in the country's east, triggering airstrikes by the U.S.-led coalition.

The Britain-based Syrian Observatory for Human Rights says 12 IS gunmen attacked the Kurdish-led Syrian Democratic Forces and clashed with them for several hours until most of the attackers were killed early Saturday.

Other activist collectives, including the Step news agency, reported the attack, saying some of the attackers used motorcycles rigged with explosives.

The fighting was concentrated near al-Omar field, Syria's largest.

The SDF, backed by U.S. air power, has driven IS from large swaths of territory it once controlled in northern and eastern Syria, confining the extremists to a small pocket of land near the border with Iraq.