US-backed Syrian fighters are preparing an offensive against the Islamic State group in eastern Syria along the border with Iraq in a race with government forces marching in the same direction against the extremists in their last major holdout in Syria.

The duelling battles for Deir el-Zour highlight the importance of the oil-rich eastern province, which has become the latest epicentre of the international war against the Islamic State group, raising concerns of an eventual clash between the two sides.

The race to reach the Iraqi border will also 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 enjoys wide influence in northeastern Syria where hundreds of American troops and advisers are helping the predominantly Kurdish Syrian Democratic Forces, one of the most effective in fighting IS in Syria.

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 percent 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.

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.

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 southeast of Raqqa near the Iraqi border.

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 U.S.-backed fighters have been gathering and stepping up preparations for an attack.

A US-trained group, the Deir el-Zour Military Council, which is part of the SDF, is expected to launch the attack against IS in Deir el-Zour under the cover of airstrikes by the US-led coalition within days. SDF officials say the imminent attack is not related to government forces reaching the city earlier this week, and was planned months in advance.

Syrian Kurdish official Nawaf Khalil, who is in Germany but frequently visits northern Syria, said the SDF attack on Deir el-Zour could begin at any moment, adding that the battle for Raqqa now no longer needs a large number of fighters.

"Deir el-Zour is a main connection point and a very important geographic area," Khalil said referring to the province linking several Syrian regions with western Iraq.

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 U.S. 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 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 Col. Ryan Dillon in June referring to a town on the Iraqi border, and using a different acronym for IS.

IBTPOTY2017 Pictures of the week
A displaced Syrian youth who fled Islamic State's Syrian stronghold of Raqqa walks on the rubble of a destroyed building in the town of Tabqa, about 55 kilometres (35 miles) west of the embattled city, on 6 September 2017. Delil Souleiman/AFP

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 IS 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.

Ahmed Abu Khawla, the commander of the Deir el-Zour Military Council, says he commands a force of 4,000 fighters, mostly from Deir el-Zour province.

"We are an organised army. We are not militias or separate brigades. We have a unified military leadership and an operations room to coordinate," he told the AP.

"The plans for the Deir el-Zour campaign have been in the works for over a year and half but Raqqa took precedence because of international considerations," said Abu Khawla.

Abu Khawla said his group has already liberated 93 villages in northwestern rural Deir el-Zour including, more recently, the village of Abou Khashab. Asked about potential confrontations with government troops, he said: "If the regime wants a confrontation or directs one bullet at us we will respond."

He also said that the SDF is already forming a local civilian council to administer the area after the military operations.

Ahmad al-Ahmad, who heads the opposition's Syria Press centre, said the SDF does not have the manpower to control Deir el-Zour, adding that government forces have brought in lots of troops and Iranian-backed gunmen for the battle.

"The regime wants to reach the border with Iraq to open a land line to Iran through Baghdad," al-Ahmad said, adding that they are capable of doing that.

By Bassem Mroue, Associated Press