BEIRUT (Reuters) – Islamic State has seized new territory from Syrian rebels in northern Syria, advancing in an area where Turkey and the United States are planning to open a new front against the group in coordination with insurgents on the ground.

The ultra-radical Islamist group and a monitor said it had seized several villages as it stepped up an offensive in northern Aleppo province, in a blow to rebels who are likely partners for Ankara and Washington in any ground campaign.

In an attack that began on Wednesday night, Islamic State had also mostly encircled the rebel-held town of Marea, some 20 km (12 miles) from the border, the group and a rebel commander in the area said.

“If its progress continues, the northern countryside of Aleppo could fall,” the rebel told Reuters. “If Marea falls, it means the fall of an important symbol of the groups fighting Daesh,” he said, using an Arabic acronym for the group.

The Syrian Observatory for Human Rights, a UK-based group tracking the war, said IS had wrested control of five villages including two near the Turkish border from other Syrian insurgents.

Turkish Foreign Minister Mevlut Cavusoglu told Reuters on Monday that his country and the United States, two NATO allies, would soon launch “comprehensive” air operations to flush Islamic State fighters from the border region.

The United States and Turkey plan to provide air cover for what Washington judges to be moderate Syrian rebels, in a joint operation to flush Islamic State from a rectangle of border territory roughly 80 km (50 miles) long. U.S. jets have already begun air strikes from Turkish bases in advance of the campaign.

Diplomats familiar with the plans say cutting Islamic State’s access to the Turkish border, across which the radical insurgent group has been able to bring foreign fighters and supplies, could be a game-changer.

CHEMICAL AGENTS

The villages captured by Islamic State on Thursday include two that the al Qaeda-linked Nusra Front recently handed over to another Syrian rebel group.

Islamic State has escalated attacks against Syrian rebels in the northern Aleppo countryside since Turkey announced plans to drive it from the area.

It has been shelling Marea, from where civilians were fleeing on Thursday towards areas closer to the Turkish border, an activist said.

International medical charity Medecins Sans Frontieres said on Tuesday it had treated a Syrian family from Marea who suffered symptoms of exposure to chemical agents.

The Syrian Observatory said rebels accused IS of using “gasses” in their offensive, but did not elaborate.

Islamic State used poison gas in attacks against Kurdish-controlled areas of northeastern Syria in late June, a Syrian Kurdish militia and a group monitoring the Syrian conflict said in July.

A U.S. general said last Friday that fragments from mortars fired by Islamic State militants at Kurdish fighters in northern Iraq had tested positive in a U.S. military field analysis for sulfur mustard, a chemical weapons agent.

Warplanes believed to be from the US-Turkish campaign were on Thursday bombing IS positions in one of the villages the militants had seized, the Observatory reported, although a rebel commander said this was not the case.

The Nusra Front, which is hostile to Islamic State, announced earlier this month that it would withdraw from the area where Turkey plans to establish a buffer zone, ceding it to other rebels.

A second group of rebel fighters trained in Turkey by the U.S.-led coalition could be deployed to Syria within weeks as part of the campaign against IS, diplomatic sources told Reuters last week.

The Nusra Front said late last month it had detained some of a first group of less than 60 U.S.-trained rebels in northern Syria weeks after they were deployed, and warned others to abandon the program.

(Reporting by Tom Perry and Naline Malla in Beirut, and Omar Fahmy in Cairo, writing by John Davison, editing by Dominic Evans)