The Islamic State terrorist group (ISIS) pushed back an offensive by US-trained Syrian rebels on a key route linking jihadist territory in eastern Syria to Iraq, a monitor said Wednesday.
The New Syrian Army, backed by US-led coalition strikes, had advanced overnight on ISIS territory near the Albu Kamal border crossing and adjacent town.
The Syrian Observatory for Human Rights said the NSA had seized the small Al-Hamdan airbase nearby, but ISIS had recaptured it by Wednesday afternoon.
"The attack failed. They lost control of the airport," said Observatory head Rami Abdel Rahman.
He said NSA units were still inside the oil-rich Deir Ezzor province where Albu Kamal lies, but had been forced to retreat.
NSA spokesman Muzahem al-Sallum confirmed that his group was no longer in control of the airport, but said it was preparing "the next phase" of their offensive.
"We retreated towards the desert around Albu Kamal," he told AFP.
NSA fighters had announced the operation on Tuesday and said it was aimed at severing ISIS's access route between eastern Syria and Iraq's western Anbar province.
The rebels said the operation was coordinated with Iraqi forces who were advancing on the crossing from the other side of the border.
The US-led coalition announced it had carried out eight air strikes near Albu Kamal and five near Al-Qaim, on the Iraqi side, on Tuesday.
ISIS seized the Albu Kamal crossing in mid-2014, when it overran swathes of territory on both sides of the border and declared a self-styled "caliphate."
NSA fighters were trained in Jordan by US and British troops.
They already captured the Al-Tanaf border crossing between Syria and Iraq earlier this year.
ISIS is facing growing pressure from US-backed offensives in both Syria and Iraq.
In northern Syria, the Syrian Democratic Forces alliance of Kurdish and Arab fighters has edged into the ISIS stronghold of Manbij with coalition air support.
In Iraq, authorities declared at the weekend that they were in full control of the city of Fallujah, west of Baghdad, long an emblematic bastion for ISIS.
AFP contributed this report.