BEIRUT — Syria’s Western-backed opposition on Saturday called for U.S. airstrikes against the Islamic State extremist group as the jihadists captured three northern villages, putting them within striking distance of a mainstream rebel stronghold.

Islamic State fighters have overrun nearly a dozen towns and villages in Aleppo province this week, crushing what little resistance they have encountered. Those gains threaten rival factions’ supply lines to neighboring Turkey and squeeze the mainstream armed opposition’s position in the country’s largest city, Aleppo, which is also under assault by President Bashar Assad’s forces.

With the mainstream rebels’ foothold in Aleppo becoming increasingly precarious, the Syrian National Coalition urged the international community to “quickly support the Free Syrian Army with weapons and ammunition” so it could “defend its people.”

“We call on the international community to use the American air force, or those of any other country to support the Free Syrian Army,” the coalition said.

The coalition has long appealed for more robust military support from the West to help in its fight to oust Assad, and more recently to counter the rise of Islamic extremists.

But the group’s appeal Saturday appeared aimed at capitalizing on the recent U.S. aerial intervention in neighboring Iraq, where American military aircraft have targeted the Islamic State group as the militants advanced.