The Islamic State of Iraq and the Levant (ISIL) group has pushed back an offensive led by US-trained rebels targeting a key route linking eastern Syria to Iraq, according to a monitor and rebel sources.
The New Syrian Army, backed by US-led coalition air strikes, had made significant advances into ISIL-held territory near the al-Bukamal border crossing, seizing the al-Hamdan airbase in the process, but ISIL fighters had recaptured the area out by Wednesday afternoon.
"The attack failed. They lost control of the airport," said Rami Abdel Rahman, the head of the Britain-based Syrian Observatory for Human Rights.
The observatory chief said that US-backed rebels were still inside Deir Az Zor, but had been forced to give up the recently gained territory.
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An anonymous rebel source told Reuters that ISIL fighters had surrounded the US-trained fighters in a surprise ambush, inflicting heavy casualties and seizing weapons.
"The news is not good. I can say our troops were trapped and suffered many casualties and several fighters and even weapons were taken," he said.
The ISIL-affiliated Amaq news agency reported earlier in the day that the group had killed 40 rebel fighters and taken at least 15 prisoner during a counteroffensive at the Hamadan airbase northwest of the city.
The New Syrian Army's offensive aimed at capturing al-Bukamal was coordinated with Iraqi government forces who were advancing towards the crossing on the other side of the border.
A spokesman for the Pentagon-sponsored rebels told the Associated Press that fighters had been dropped from coalition helicopters on al-Bukamal's southern edge to assist in the operation.
The US-led coalition said it had conducted eight air strikes near al-Bukamal and five near al-Qaim, on the Iraqi side of the border.
ISIL seized the border crossing in mid-2014, as it overran swaths of the oil-rich Deir Az Zor province, effectively erasing the demarcation line between Syria and Iraq.
A rebel takeover of the border crossing would be a heavy blow to ISIL's self-declared, cross-border "caliphate", effectively splitting its territorial holdings in two and preventing the transfer of fighters and weapons between the two countries.
The US-led coalition has stepped up its campaign against the hardline group this month. An alliance of US-backed militias, including the Kurdish YPG, launched a separate offensive against the group earlier in the month on the ISIL-held city of Manbij in northern Syria.
The Iraqi government declared victory over the weekend in its fight against the group in Fallujah, taking control of the city after nearly two years of rule by ISIL, also known as ISIS.
The New Syrian Army has received military training in US-run camps in Jordan, but according to rebel sources, most of their training is now being conducted at a base in al-Tanf, a border town southwest of al-Bukamal, which the rebels captured from ISIL earlier this year.
Al-Tanf was hit twice earlier this month by Russian air strikes.