By Tom Perry

BEIRUT (Reuters) - Syrian rebels, who this week seized a strategic town from Islamic State, aim to move westward in the next phase of their Turkey-backed operation, an advance that could take weeks or months to complete, a rebel commander said.

Colonel Ahmad Osman, head of the Sultan Murad group, also told Reuters the rebels did not wish to fight Kurdish forces that have advanced in northern Syria as part of a separate campaign against IS, but would do so if necessary.

Sultan Murad is one of the main Syrian rebel groups taking part in the operation that on Wednesday drove IS from the border town of Jarablus with help from Turkish special forces, warplanes and tanks.

The operation aims to expel IS from its last foothold at the Syrian-Turkish border, but also to prevent any further gains by the Kurdish YPG militia whose growing influence in Syria has alarmed Turkey as it battles its own Kurdish insurgency.

Osman, speaking to Reuters from Jarablus, said the priority was now to advance some 70 km (40 miles) westward to Marea, a town where rebels have long had a frontline with IS.

"We want to cleanse this area before moving south," Osman said. Failing to do so would leave the rebels at risk of IS counter attack, he said. "The priority is from Jarablus in the direction of al-Rai, reaching Marea."

Al-Rai is a village between Jarablus and Marea captured by rebels this month.

Osman said there were dozens of villages between Jarablus and Marea that must be recaptured from IS. "Liberating these villages between Jarablus and Marea requires weeks and perhaps months, according to the nature of the battles," he said.

Several rebel groups are taking part in the operation, fighting under the banner of the Free Syrian Army. They have received varying degrees of help from Syrian President Bashar al-Assad's foreign enemies, including military aid and training.

Osman, 49, said both Turkey and the U.S.-led coalition against Islamic State, of which Ankara is a part, were keen to provide air support for the operation.

The rebel factions have also advanced southward from Jarablus toward the city of Manbij, which was captured earlier this month from Islamic State by the U.S.-backed Syrian Democratic Forces (SDF) alliance including Kurdish fighters.

The SDF crossed the Euphrates river in order to attack Manbij. Turkey has demanded the Kurdish YPG fighters now go back across the river.

The rebels fighting with Turkish support are hostile to the YPG, with the sides clashing repeatedly since last year. "We are currently planning not to confront them, but if we have to confront them, we will," Osman said.

Osman, who defected from the Syrian military to join the uprising in 2012, said that for some operations, the number of rebel fighters may need to be increased from its current level of between 1,200 and 1,500.

"We are ready to supply greater numbers, depending on the nature of the battle," he said.

(Writing by Tom Perry, editing by G Crosse)