BEIRUT — US-backed Syrian fighters launched an offensive against the Islamic State group in eastern Syria along the border with Iraq on Saturday, bringing them into a race with government forces against the extremists in their last major holdout in the war-torn country.

The dueling 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 the Islamic State group, raising concerns of an eventual clash between the two sides.

The US-trained Deir el-Zour Military Council is a part of the predominantly-Kurdish Syrian Democratic Forces. The SDF are the US’s primary ally in fighting the Islamic State 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 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 the Islamic State. Three months into the battle, they have liberated around 60 percent of the city, and much more difficult urban fighting lies ahead.

This week, Syrian troops and their Iranian-backed allies reached Deir el-Zour, breaking a nearly 3-year-old Islamic State siege on government-held parts of the city in a major breakthrough in their offensive. In a victory statement, the Syrian military said Deir el-Zour will be used as a launching pad to liberate the remaining 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, 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 barren coffers.

Syria’s military command announced Saturday they had captured the province’s Taym oil field from militants on Saturday.

Washington has been determined to block the formation of an ‘‘Iranian corridor’’ — of Shi’ite-controlled land stretching from Tehran to Damascus — and for months has been eyeing the area southeast of Raqqa.

US-backed Syrian rebels had been gathering in Tanf in southeastern Syria to march toward 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 northeastern province of Hassakeh, where Syrian activists say the US-backed fighters have been stepping up preparations for an attack.

SDF officials say the timing of their offensive was not related to government forces reaching the 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.

US Defense Secretary Jim Mattis said last month during a visit to the Middle East that the Middle Euphrates River Valley will soon be liberated, as the Islamic State takes hits from both sides of the valley that bisects Iraq and Syria. ‘‘You see, ISIS is now caught in between converging forces,’’ Mattis said.