Syrian government troops and allied forces reached the Iraqi border for the first time since 2015 on Friday, a monitor said, risking fresh tensions with US-led forces nearby.
The Syrian Observatory for Human Rights, a Britain-based monitor, said dozens of pro-regime forces had arrived at the point on the Syria-Iraq border around 70 kilometres north of At-Tanaf, which is on the Syria-Iraq border.
There was no immediate confirmation of the report in Syrian state media.
Forces from the US-led coalition fighting the Islamic State group in Syria and Iraq are using a garrison in At-Tanaf to stage attacks against the jihadists and train Syrian rebels to fight them.
The area has been the scene of repeated incidents between US-led forces and pro-regime troops, including on Thursday when a US warplane shot down a pro-regime drone near the area.
The shoot-down came after another incident earlier Thursday in which coalition forces struck "technical vehicles" advancing toward At-Tanaf and threatening coalition and partner forces, the US Central Command said. The incident was the third time US-led forces have struck pro-regime forces near At-Tanaf in less than a month.
The coalition said it shot down the drone after it fired at US-led forces in the area.
The coalition has established a "de-confliction" zone, extending 55 kilometres (34 miles) from the ga rrison, in which pro-regime and Russian forces are not supposed to operate.
Earlier in the week, coalition forces "destroyed" a pro-regime unit that was moving into the area with a tank, artillery, anti-aircraft weapons and armed technical vehicles. And on May 18, coalition planes struck a convoy that had apparently been headed toward At-Tanaf.
Syria's government has been eager to reach the eastern border after recapturing territory in central Homs province once held by the Islamic State group.
Damascus is hoping to lead the battle against the jihadist group in the eastern province of Deir Ezzor and head off any attempts by the US-led coalition to do so.
"It's a question of refusing to allow the US and the forces it supports to occupy the country's east," Wadah Abed Rabbo, editor-in-chief of the Al-Watan daily which is close to Syria's government, told AFP last month.
Syria's government has not held positions on the border with Iraq since 2015, when IS fighters swept down from neighbouring Deir Ezzor and captured positions in Homs province, including the ancient city of Palmyra.
Regime troops have since recaptured large swathes of territory in central Syria from the jihadists.
The group is also under attack in the northern city of Raqa, its onetime bastion, with a US-backed alliance of Kurdish and Arab fighters leader the battle.