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 IS and a monitoring agency said the group 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.
Intense attacks began overnight and on Thursday morning IS fighters had mostly encircled the rebel-held town of Marea, some 20 km (12 miles) from the Turkish border, a rebel leader fighting against the group in the area said.
Another insurgent commander battling Islamic State said his men were fighting back in earnest to stop the jihadists from taking Marea, in clashes that had killed at least two dozen fighters on each side.
IS used "toxic gases" while shelling the town, he said.
Medical charity Medecins Sans Frontieres recently said it had treated a family from Marea suffering symptoms of exposure to chemical agents.
The assault on Marea could prove disastrous for rebels fighting Islamic State, which has overrun swathes of Iraq and Syria over the past year.
"If its progress continues, the northern countryside of Aleppo could fall," one 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 from other insurgents, including two near the Turkish border.
Turkish Foreign Minister Mevlut Cavusoglu told Reuters on Monday his country and the United States would soon launch "comprehensive" air operations to flush IS fighters from the border region.
The two NATO allies plan to provide air cover for what Washington judges to be moderate Syrian rebels, in a joint operation to drive IS 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 IS access to the Turkish border, across which the radical group has been able to bring foreign fighters and supplies, could be a game-changer.
Warplanes believed to be from the U.S.-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 villages captured by IS on Thursday include two that the al Qaeda-linked Nusra Front - which is hostile to IS - recently handed over to another Syrian rebel group after announcing it was withdrawing from the intended Turkish buffer zone area.
Nusra Front has warned other rebels not to be distracted from the fight against Syrian President Bashar al-Assad by getting bogged down in potentially costly battles with IS.
(Reporting by Tom Perry and Naline Malla in Beirut, and Omar Fahmy in Cairo, writing by John Davison, editing by Dominic Evans and Gareth Jones)