BEIRUT (AP) -- Islamic State militants on Wednesday pushed back U.S.-trained Syrian rebels from the outskirts of a town on the Iraqi border, dealing a setback to a budding offensive that aims to sever the militants' transit link between Syria and Iraq, a rebel spokesman said.
The Islamic State-linked Amaq news agency said IS militants repelled the New Syrian Army from an air base which the rebels had briefly captured earlier in the day. IS also claimed it had seized 15 hostages.
Earlier Wednesday, the Pentagon-trained force entered the Hamdan air base -- northwest of the border town of Boukamal -- following intense clashes, rebel spokesman Mozahem al-Saloum said.
The base was used as a militant outpost for the IS-held town. Airborne fighters were dropped from coalition helicopters on Boukamal's southern edge, helping the fighters advance, said al-Saloum and the Britain-based Syrian Observatory for Human Rights.
The Observatory's chief, Rami Abdurrahman, said foreign airborne fighters were also dropped, but to the north, enabling the takeover of the base. The rebels were heavily backed by U.S.-led coalition airstrikes and were coordinating their fight with Iraqi tribesmen and forces on the other side of the border, al-Saloum said.
The Observatory said several rebel factions were involved in the offensive, bringing ground troop numbers to several hundred. It said IS fighters have dug trenches south of the town and planted land mines to prevent advances.
A spokesman for the Joint Military Operation Command denied knowledge of the operation.
The new offensive was launched Tuesday. Amid the heavy fighting, al-Saloum said the New Syrian Army forces were unable to keep the base and other outposts to the south, near the Qaim border crossing with Iraq.
"We can't continue," he said. But al-Saloum said the retreat was only the end of phase one.
Wednesday's setback was another blow to the NSA. Two weeks ago, Washington accused Russian aircraft of bombing the NSA near the Iraqi border. Russia has been carrying out airstrikes in support of Syrian President Bashar Assad's forces since September.
IS seized much of the Iraq-Syria border in its 2014 blitz, along with large swaths of territory in both countries, declaring an Islamic caliphate. But IS has in recent weeks been losing ground, both in Iraq and in Syria.
U.S.-backed Kurdish-led forces have besieged Manbij, an IS stronghold in northern Syria's Aleppo province, while Iraqi forces have taken Fallujah in Iraq's western Anbar province from the Sunni extremist group.