BEIRUT/AMMAN (Reuters) - Syrian rebels are preparing to launch an attack to seize a town from Islamic State on the border with Turkey, a senior rebel said on Sunday, in a move that would frustrate Kurdish hopes to expand in the area.

An Islamic State flag flies over the custom office of Syria's Jarablus border gate as it is pictured from the Turkish town of Karkamis, in Gaziantep province, Turkey August 1, 2015. REUTERS/Murad Sezer

The rebels, Turkish-backed groups fighting under the banner of the Free Syrian Army (FSA), are expected to assault Jarablus from inside Turkey in the next few days, said the rebel official, who is familiar with the plans but declined to be identified.

“The factions are gathering in an area near the border (inside Turkey),” the rebel said.

Another rebel source said they were gathering at a Turkish military camp near the town of Qarqamish just opposite Jarablus.

“Every day there are groups of fighters entering from inside Syria across a secret crossing to a Turkish base where they are gathering in preparation of the assault on Jarablus,” the source said.

Fighters mostly drawn from Failaq al Sham, Sultan Murad, Ahrar al-Sham and Jabha al Shamiya groups were coming from Syria’s northwestern rebel-held Idlib province and also from the town of Azaz.

Another source, in Ahrar al-Sham, expected the assault on Jarablus to begin in the next few days.

Jarablus, on the western bank of the Euphrates river, is the last significant town held by the militant Islamist group on Syria’s border with Turkey.

It is 34 miles (54 km) east of al-Rai, a border town FSA rebel groups recently took from Islamic State.

RIVALRY WITH KURDS

By taking Jarablus themselves, the rebel groups would preclude an assault on the town by the Syrian Democratic Forces (SDF), a group of Kurdish-dominated militias who on Aug. 6 took the city of Manbij, 20 miles (30 km) to the south, from IS.

A group allied to the SDF and calling itself the Jarablus military council issued a statement on Sunday accusing Turkey of supporting radical jihadist groups and calling on the U.S.-led coalition to back the council.

The U.S.-led coalition has helped the SDF to make significant gains against Islamic State militants in northern Syria since it was established last year.

Turkey, an important supporter of the FSA groups, is worried that Kurds are using the SDF’s westwards expansion against IS to extend their influence across northern Syria. The SDF already holds the eastern bank of the Euphrates opposite Jarablus.

On Saturday Turkish Prime Minister Binali Yildirim said that Ankara would play a more active role in addressing the conflict in Syria in the coming six months to stop it being torn along ethnic lines.

The rebel source from Ahrar al Sham said Turkish artillery had since Friday been pounding several Islamic State positions in villages on the outskirts of Jarablus near the border strip.

The mainly Sunni Arab rebel groups are now aiming to regain control of the strategic towns of Tal Rifaat and Marea in the northern Aleppo countryside, currently in the hands of the YPG, the powerful Kurdish militia.

Islamic State has pulled personnel out of Jarablus in recent days, the rebel leader said. On Friday families of IS fighters were evacuated from Jarablus and another city nearby, al-Bab, to the group’s stronghold of Raqqa, the Syrian Observatory for Human Rights said.

The rebel operation aimed to effectively end Islamic State’s presence on the Turkish border, the official said.

“There will certainly be resistance. They will have mined it heavily,” he said. “The operation of entering Jarablus will not be easy.”