US-backed Syrian fighters have launched an offensive against the Islamic State group in eastern Syria along the border with Iraq.
The move has brought them into a race with Syrian government forces marching in the same direction against the extremists in their last major holdout in the war-torn country.
The duelling battles for Deir el-Zour highlight the importance of the oil-rich eastern province, which has become the latest focus of the international war against IS, raising concerns of an eventual clash between the two sides.
The US-trained Deir el-Zour Military Council said it was calling its operation Jazeera Storm, after the familiar name for north-east Syria.
The Military Council is a part of the predominantly-Kurdish Syrian Democratic Forces (SDF), which enjoys broad US military support.
The SDF are the US's primary ally in the fight against IS in Syria.
The race to reach the Iraqi border will shape future regional dynamics, determining whether the United States or Russia and Iran will have more influence in the strategic area once the extremist group is defeated.
Iran has been one of President Bashar Assad's strongest backers since the crisis began in March 2011 and has sent thousands of Iranian-backed fighters and advisers to fight against insurgent groups trying to remove him from power.
The US-backed fighters are up against a huge challenge to reach Deir el-Zour, especially while they are still fighting to liberate Raqqa from IS.
Three months into the battle, they have liberated around 60% of the city, and much more difficult urban fighting still lies ahead.
This week, Syrian troops and their Iranian-backed allies reached Deir el-Zour, breaking a nearly three-year-old IS siege on government-held parts of the city in a major breakthrough in their offensive against IS.
In a victory statement, the Syrian military said Deir el-Zour will be used as a launching pad to liberate the remaining IS-held areas along the border with Iraq.
Pro-government forces broke the siege of the city's airport on Saturday, state media reported.
The troops' arrival to Deir el-Zour city brings Syrian forces and their allies a step closer to controlling the oil-rich eastern province and its capital bordering Iraq, a major boost for Tehran's growing influence in the area.
The region has some of Syria's largest oil fields, whose revenue is vital to the state's dried coffers.
Syria's military command announced on Saturday they had captured the province's Taym oil field from IS militants on Saturday.
Washington has been determined to block the formation of an "Iranian corridor" - of Shiite-controlled land stretching from Tehran to Damascus - and for months has been eyeing the area south-east of Raqqa near the Iraqi border.
US-backed Syrian rebels had been gathering in Tanf in south-eastern Syria to march towards Deir el-Zour, but their plans were disrupted in June when Syrian troops reached the border with Iraq, obstructing their path.
The only way left for the SDF to enter the eastern province appears to be from the north-eastern province of Hassakeh, where Syrian activists say the US-backed fighters have been gathering and stepping up preparations for an attack.
SDF officials say the timing of Operation Jazeera Storm is not related to government forces reaching Deir el-Zour earlier this week, and was planned months in advance.
"Deir el-Zour is a main connection point and a very important geographic area," said Syrian Kurdish official Nawaf Khalil, who is in Germany but frequently visits northern Syria.
He said the battle for Raqqa requires fewer fighters now than it did in its earlier stages.
The US-led coalition fighting IS said in an email to The Associated Press that the SDF "will decide when the conditions are right for an offensive".
Asked about concerns of a possible clash between the SDF and Syrian troops, the coalition said: "We urge all forces to concentrate their efforts on our common enemy (IS)."
Washington has welcomed Syrian troops' fight against IS. Both the US and Russia have an interest in avoiding a clash between the SDF and Syrian forces and may devise a strategy that will allow both sides to share control of the vast province.
US officials have suggested they are not seeking a confrontation with Mr Assad's forces.
"We are in the killing-Isis business. That is what we want to do, and if the Syrian regime wants to do that... and show that they are doing just that in Abu Kamal or Deir el-Zour or elsewhere, that means that we don't have to do that in those places," said coalition spokesman Colonel Ryan Dillon in June referring to a town on the Iraqi border.