BEIRUT (Reuters) - Islamic State fighters withdrew from frontlines with Syrian rebel forces north of Aleppo on Wednesday as they mounted a counter attack against the jihadist group near the Turkish border, an opposition source and monitoring group said.

The sudden withdrawal from villages around the rebel-held town of Marea points to the pressure Islamic State is feeling from offensives being waged by other enemies further east, the Syrian Observatory for Human Rights monitoring group said.

Islamic State had managed to besiege the rebel-held town of Marea in a significant advance late last month, stranding thousands of civilians there and prompting a U.S.-led coalition to air drop weapons to rebels, rebel sources said.

Rebel fighters in Marea broke the siege on Wednesday when they captured the village of Kafr Kalbin on the road linking Marea with Azaz, 20 km (12 miles) to the northwest at the border with Turkey. The advance was preceded by a rebel statement saying they were uniting their ranks.

"It seems they (IS) can't keep several fronts open at the same time. It is a strategic area, they were on the verge of entering Azaz," Observatory Director Rami Abdulrahman said. The opposition source said Islamic State had withdrawn from the area quickly, and Free Syrian Army factions had filled the void.

FSA rebels fighting Islamic State north of Aleppo have received military assistance from states opposed to President Bashar al-Assad.

Their battle with Islamic State is separate to one being waged further east by a U.S.-backed group, the Syria Democratic Forces, which includes the Kurdish YPG militia. The Syrian army, backed by Russian air strikes, has also advanced against Islamic State since last week.

The FSA rebels are fighting separate conflicts with both the SDF and Assad, their main enemy.

(Writing by Tom Perry; Editing by Andrew Heavens and Peter Graff)