Syrian president Bashar al-Assad's forces and allied militias reached the Iraqi border this weekend -- advancing on, and effectively cutting off, a base used by Syrian rebels and US-led coalition forces training them to fight Isis.
The advance comes amid escalating tensions over the past week between regime forces, backed by Mr Assad's international patrons Iran and Russia, and the US-backed rebels.
The US twice last week struck regime forces, which it said were advancing into a "safe zone" it had established around the al-Tanf base where they are training and operating with rebels. The base is located near eastern Syria's borders with Iraq and Jordan.
On paper, the rival Russian and US-backed alliances are both fighting the jihadi group Isis, whose final footholds are in eastern Syria. But observers have dubbed their efforts a "race for eastern Syria". International powers are struggling for control of the resource-rich region, most notably because it could allow Iran to create an overland supply route to all of its proxies across the region, which makes the US and its allies wary. For weeks, rebels had complained that Iranian-backed Shia militias were pouring into the area, including some forces from Hizbollah, the powerful Lebanese force.
On Saturday, Syria's armed forces announced their advance to the Iraqi border, north of al-Tanf, and called it a critical step in their battle against Isis. They also sent a warning to western forces in the area.
"The general command of the army and armed forces warn of the risks of repeated attacks of the so-called 'international coalition' and their efforts to block the advances of the Syrian Arab Army and its allies in their war on terror," the army said in a statement cited by state news agency SANA.
Regional diplomats say that Russian officials had previously seemed wary of intervening in the internationalised scramble for eastern Syria. But in the past week, they said Russian counterparts in Damascus expressed support for the campaign by regime and militia forces and warned western counterparts that Mr Assad's forces would advance on Tanf itself.
Sergei Lavrov, Russia's foreign minister, told US secretary of state Rex Tillerson on Saturday that US strikes on pro-government forces in Syria were unacceptable, calling for "concrete measures to prevent similar incidents in the future", according to a statement by Russia's foreign ministry.
Syria's six-year civil war began as protests against Mr Assad but has spiralled into a multi-sided conflict. The chaos has given room for jihadi militants such as Isis to seize swaths of territory and has dragged in international powers into the fray, with Russia and Iran acting as Mr Assad's main backers. Washington, along with Turkey and Gulf Arab states, support some rebel groups, but are mostly preoccupied with the fight against Isis.
For the US-led coalition, the pro-government forces' advance north of Tanf will now block a ground campaign they had been preparing to push into the Isis-held city of Al-bukamal. It was part of an effort to rout Isis from the eastern province of Deir Ezzor. As Isis loses territory to international forces advancing on all sides, the oil-rich area is expected to be one of the jihadi group's final territorial strongholds.