Deployment raises fears about possibility of fresh clashes between US and Syrian forces
Syrian government troops and allied forces on Friday reached the Iraqi border for the first time since 2015, a monitor said, risking fresh tensions with US-backed forces nearby.
The Syrian Observatory for Human Rights, a Britain-based monitor, said dozens of pro-government forces had arrived at the Syria-Iraq border about 70km north of At-Tanaf.
There was no immediate confirmation of the report in Syrian state media.
Forces from the US-led coalition fighting the Islamic State (IS) group in Syria and Iraq are using a garrison in At-Tanaf to stage attacks against the militants and train Syrian rebels to fight them.
The coalition did not comment specifically on the reported advance.
But when asked about it by AFP, it said "the demonstrated hostile intent and actions of pro-government forces near coalition and partner forces in southern Syria ... continue to concern us and the coalition will take appropriate measures to protect our forces".
"As long as pro-regime forces are oriented toward coalition and partnered forces the potential for conflict is escalated," it added in a statement.
The Pentagon credited Syria government ally Russia with helping to calm tensions.
"Russia has been very helpful, and I think that the calm we see today is largely due to their efforts," spokesman Navy Captain Jeff Davis said on Friday.
"They are trying to get the other parties, the pro-government, the Iranian-backed militias to do the right thing and to prevent them from taking actions that are destabilising."
The Russians "have helped to pass messages and calm the situation there, and we hope that continues," he said.
The At-Tanaf area has been the scene of repeated incidents between US-led forces and pro-government troops, including on Thursday when a US warplane shot down a pro-government drone near the area.
The drone dropped what turned out to be a dud bomb.
The Pentagon has not said who was operating the drone, but the At-Tanaf area has seen a surge in activity by Iran-backed troops loyal to Syrian President Bashar al-Assad.
Iran makes an armed drone similar to the one shot down.
US forces have also conducted three strikes against pro-government forces it deemed to be threatening At-Tanaf.
On Thursday, coalition forces struck "technical vehicles" advancing towards At-Tanaf and threatening coalition and partner forces, the US Central Command said.
The coalition has established a "de-confliction" zone extending 55km from the garrison, in which pro-government and Russian forces are not supposed to operate.
Earlier in the week, coalition forces "destroyed" a pro-government unit that was moving into the area, and on 18 May coalition planes struck a convoy that had apparently been heading towards At-Tanaf.
Syria's government has been eager to reach the eastern border after recapturing territory in central Homs province once held by IS.
Damascus is hoping to lead the battle against the militants in the eastern province of Deir Ezzor and head off any attempts by the US-led coalition to do so.
Syria's government has not held positions on the border with Iraq since 2015, when IS swept down from neighbouring Deir Ezzor and captured positions in Homs province, including the ancient city of Palmyra.
Government troops have since recaptured large areas of territory in central Syria from the militants.
The group is also under attack in the northern city of Raqqa, its one-time bastion, with a US-backed alliance of Kurdish and Arab fighters leading the battle.