Syrian fighters have cut off the main road between the northern Syrian city of Raqqa and the eastern city of Deir el-Zour as they press against Islamic State (IS) territory.
Despite the fresh blockade on the Raqqa-Deir el-Zour road, IS still controls large areas south of Raqqa, and the extremists should be able to use smaller roads and paths through open desert to move between the cities.
Fighters from the Kurdish-led Syria Democratic Forces (SDF) have been on the offensive under the cover of US-led coalition air strikes since November.
Operation Euphrates Wrath aims to surround and capture Raqqa, the de facto capital of the IS extremists' self-declared caliphate.
The SDF began the third phase of Euphrates Wrath last month with the aim of capturing villages and towns east of Raqqa.
The UK-based Syrian Observatory for Human Rights said SDF fighters had cut off the road linking the two cities in Jazra near Raqqa.
SDF spokesman Talal Sillo said "our forces have cut the main supply line between Raqqa and Deir el-Zour".
He added that it is still too early to impose a siege on Raqqa since the extremists still control areas west and south of the city.
The blockade of the main road comes amid an ongoing SDF march toward Raqqa. Its fighters are now stationed five miles north of the Euphrates River city and supported by both coalition air strikes and a deployment of some 500 US special forces operators.
The Pentagon has said its soldiers are working in a purely advisory capacity.
Turkey, a US ally through Nato, views the Kurdish militia known as the YPG - the main component of the SDF - as an extension of the Kurdish insurgency inside its own borders, and has classified the party as a terror organisation.
It has objected strongly to the SDF offensive and also vowed to repel the Kurdish-led forces in the town of Manbij back over the banks of the Euphrates, a move which would disrupt the Raqqa campaign.
There are Turkish forces stationed in al-Bab, 25 miles south-west of Manbij.
The Turkish threats prompted the SDF to ask Russia and the Syrian army to establish a buffer between them and the Turkish forces.
With uncertainty building, the US deployed a number of armoured vehicles to its allies in Manbij, the Syrian Kurdish Rudaw news agency reported.
Operation Inherent Resolve spokesman Col John Dorrian confirmed the deployment on Twitter. He said it was meant to "deter aggression and keep focus on defeating ISIS", another acronym for the Islamic State group.