AMMAN (Reuters) - U.S.-backed forces said on Saturday they had seized a major natural gas field in Syria’s Deir al-Zor province from Islamic State militants in rapid advances since the start of an operation earlier this month to capture areas east of the Euphrates river.

Smoke rises as Syrian army soldiers stand near a checkpoint in Deir al-Zor, Syria September 21, 2017. REUTERS/Omar Sanadiki

Commander Ahmed Abu Khawla told Reuters that the Conoco gas field was the first of its kind taken by the Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, since it began an offensive earlier this month to capture the eastern province.

“This is the first gas or oil field in the campaign which we have liberated,” said the commander of the group whose campaign is in parallel with an ongoing battle for the nearby Raqqa city.

The Conoco field is named for the American company which discovered the gas reserves and built a processing plant there. It has been the target of U.S. strikes and was used to supply cooking gas canisters for household use. Before the conflict it supplied gas to power stations.

In Deir al-Zor, Islamic State is battling two separate offensives, one launched by the SDF and the other by the Syrian army and its allies.

Syrian troops supported by Iranian-backed militias have also crossed to the eastern side of the river, increasing their presence in an area where U.S.-backed militias have also advanced.

The assaults by the Russian-backed Syrian army and the U.S.-backed SDF have at times raised fears of clashes that could stoke tensions between the competing world powers.

Islamic State’s last major stronghold, the cities, towns and farms in the fertile strip along the Euphrates are fast becoming the focus of Syria’s war.

Abu Khawla said the Syrian army and their allies were within four km (2.5 miles) of the SDF positions.

Russia said on Thursday it had warned the United States it would target areas in Syria where U.S. special forces and U.S.-backed militia were operating if its own forces came under fire from them, something it said had already happened twice.[L5N1M235E]

The Russian warning underscored growing tensions over Syria between Moscow and Washington. While both oppose Islamic State, they are engaged, via proxies, in a race for strategic influence and potential resources in the form of oilfields in Deir al-Zor province.

The Syrian army’s command said in a statement its forces captured at least 44 villages and towns since launching two weeks ago an assault into Deir al-Zor city that ended a militant siege which had lasted three years.

On Saturday the army also announced it took Maadan town, in southern Raqqa along the provincial boundary with Deir al Zor province.

Army commanders however say militants in Hweija Sakr, southeast of the river are slowing their advance, hiding amid thick vegetation and waging hit and run ambushes and suicide bombings.

Islamic State’s Amaq newsagency said they had killed and injured at least seventy army fighters in a suicide bombing attack on an army convoy in the area.

Colonel Ryan Dillon, spokesman for the U.S.-led coalition fighting Islamic State, denied that Washington was in a race with Moscow over the oil-rich province, saying contacts with the Russian military were intensifying to avoid confrontation.

“From the coalition’s perspective it has not been a race, we are not in the land-grabbing business,” Dillon told Reuters.

U.S.-backed forces were avoiding the eastern bank of Deir al-Zor city right now where the Syrian army was fighting the jihadists and seeking to expand control in the area northeast of the river still held by the militants, Dillon said.

The eventual goal was to move toward the Iraqi border near the main militant strongholds of al-Mayadeen, about 80 km (50 miles) west of the Iraqi border and al-Bukamal, both large urban centres where the Euphrates flows through.

“The SDF are not moving toward the banks of the Euphrates river right now (near the city of Deir al-Zor). They are moving east toward the Iraqi border ... and continuing to push and expand into ISIS-held territory,” he added.

“Our goal as I have said a dozen times is fighting ISIS. We don’t have a fight with the (Syrian) regime. We are not in a fight with the Russians. We are there to fight ISIS and that’s what we are going to do,” the U.S. army spokesman said.