AMMAN (Reuters) - Syrian rebels have seized large areas from Islamic State in southern Syria in the last two weeks as the jihadist group prepares to defend its Raqqa stronghold in the north from a U.S.-backed assault, rebel commanders say.

Free Syrian Army fighters carry their weapons on the outskirts of al-Bab, February 2017. REUTERS/Khalil Ashawi

The advances by Western-backed Free Syrian Army (FSA) factions have helped to reduce the risk of Islamic State fighters regrouping in areas near Damascus and the Jordanian border as they face major defeats in Syria and Iraq.

Western intelligence sources have worried for months that militants fleeing from their main urban strongholds of Raqqa in Syria and Mosul in Iraq could find a safe haven in the vast areas of the Syrian Desert bordering Jordan.

The rebels fighting in southern Syria have received military aid funneled via Jordan in a program overseen by the U.S. Central Intelligence Agency.

Their sudden gains are a culmination of months of covert operations in which they have ambushed and cut communications lines to weaken the militants’ stronghold in the southeastern border area close to Iraq, the rebels say.

“Extensive areas have fallen into our hands. (Islamic State) has been pushed out of them in heavy clashes in 16 days of battles,” said Talas al Salameh, the commander of the Osoud al Sharqiya, the biggest of the FSA groups in the area.

“(Islamic State) had cut roads and were in control and had been positioned in former Syrian army bases with a strong presence and with heavy armor. We cut links between their areas and as a result they began to retreat,” al-Salamah said.

In northern Syria, Islamic State has come under growing military pressure in recent months from separate campaigns being waged by the U.S.-backed Arab and Kurdish fighters, by the Russian-backed Syrian army and by Turkey-backed FSA groups.

The U.S.-led coalition against Islamic State is backing an alliance of Kurdish and Arab fighters - the Syrian Democratic Forces (SDF) - in the campaign to isolate and capture Raqqa city on the other side of the country.

TROOP MOVEMENTS

Salamah said Islamic State had pulled hundreds of fighters from the areas where his group, working with four others, had made its advances, suggesting they had been redeployed to help defend Raqqa and Deir al-Zor province to the east.

The advances have taken place in a swathe of sparsely populated territory stretching from the town of Bir Qassab, some 50 km (30 miles) southeast of Damascus, all the way to the borders with Iraq and Jordan, a desert area known as the Badia.

“In the event of the fall of Raqqa and Mosul, where would they go? They would be coming here. So we decided to work and kick them out of this area before they would come to us,” Salameh told Reuters in a phone interview.

The rebels have also seized control of the eastern slopes of the Qalamoun mountains to the northwest of Bir Qassab, where Islamic State’s presence had disappeared as it moved its forces further north, he said.

The rebels say their campaign in the area had been gradually escalated over the last five months.

Said Seif, an official in another FSA faction, the Shahid Ahmad al-Abdo group, said 250 square km (96 square miles) had been captured in the Badia alone.

Salameh said at least 117 of his fighters had been killed in ferocious fighting over the last few months in what he said were relentless assaults and ambushes by the militants.

His group first fought Islamic State when it took over parts of Deir al-Zor province at the height of its expansion in 2014. His fighters regrouped in areas near the Jordanian border, where their base was hit in a Russian air strike last year.