BEIRUT (AP) — Kurdish-led Syrian fighters have launched a new advance on the northern town of Manbij, a key Islamic State stronghold, with the aid of U.S.-led airstrikes, a spokesman for the fighters and a monitoring group said Wednesday, June 1.

The United Nations meanwhile said it was looking into “every possible means” to deliver life-saving aid to besieged Syrians now that a Wednesday deadline set by world powers has passed. U.N. spokeswoman Josephine Guerrero said the “main priority” is to access 592,700 people in besieged areas and millions more in hard-to-reach areas facing severe food shortages.

The U.S.- and Russia-led International Syria Support Group last month called on the U.N. to “immediately carry out a program for air bridges and air drops for all areas in need” starting June 1 if it was denied access to designated areas. The U.N. envoy to Syria, Staffan de Mistura, has called air drops, which cost more than land delivery, a “last resort.”

The so-called Syrian Democratic Forces, a Kurdish-led militia that includes Arab fighters, have approached to within 14 kilometers (nine miles) of Manbij, which lies along a key supply route from the Turkish border to the city of Raqqa, the de facto capital of the IS group’s self-styled caliphate. The SDF announced a campaign to advance on areas around Raqqa last week.

“There are tens of casualties among fighters and evacuating civilians,” said Nasser Haj Mansour, an adviser to the SDF. He could not provide an exact figure. “Daesh is using its entire means to stop the forces,” he told the AP by telephone from near the front line, referring to IS by its Arabic acronym.

The Britain-based Syrian Observatory for Human Rights said the fighters had recaptured at least 16 villages from IS. The monitoring group, which relies on a network of activists inside Syria, said coalition airstrikes supporting the advance have killed 15 civilians in the past 24 hours.

The SDF has also launched an offensive to capture the IS-held Tabqa air base, about 40 kilometers (25 miles) southwest of Raqqa. Mansour said the air base is a “major weapons depot” for the extremist group.

IS seized Tabqa in 2014 and massacred at least 160 captured Syrian soldiers.

Turkey’s state-run news agency meanwhile said coalition airstrikes and Turkish artillery fire killed 14 IS militants near the Turkish border. The Observatory said no casualties had been inflicted.

The Anadolu Agency, citing military officials, said Wednesday that the strikes by U.S.-led coalition jets targeted IS positions north of the city of Aleppo, destroying a tank, two mortar positions, a headquarters’ building and three vehicles. It said the strikes came after Turkey’s military had determined that IS was preparing to attack Turkish territory from the region.

The report could not be independently verified. Turkey has not explained how it counts casualties in Syria.

Cross-border fire from Syria has killed 21 people and wounded dozens of others in the border town of Kilis this year.