BEIRUT (Reuters) - U.S.-backed forces in Syria will soon launch an offensive to oust Islamic State militants from Deir al-Zor province, their last major foothold in the country, an SDF official said on Friday.

Members of Deir al-Zor military council which fights under the Syrian Democratic Forces (SDF) seen in Deir al-Zor province, Syria August 25, 2017. REUTERS/Rodi Said

The Syrian Democratic Forces (SDF) could start its assault on Deir al-Zor “within several weeks” in parallel with an ongoing battle for nearby Raqqa city, Ahmed Abu Khawla told Reuters.

The SDF alliance of Kurdish and Arab militias has been fighting IS inside Raqqa since June after a months-long advance on the city, backed by air strikes and special forces from the U.S.-led coalition.

As Islamic State has come under pressure in Raqqa, many of its forces have fallen back on the towns and cities further east along the Euphrates in Deir al-Zor province.

Syrian government forces are fighting their own campaign in a different part of the province, which borders Iraq.

“The operation to liberate Deir al-Zor will begin very, very soon,” said Khawla, who heads the Deir al-Zor military council that fights under the SDF banner.

Col. Ryan Dillon, a spokesman for the U.S.-led coalition, said its focus remained on Raqqa.

Khawla said military plans were ready, and that his unit had already “entered Deir al-Zor territory and liberated several towns”.

The military council included 4,000 fighters, mostly Arabs and mostly from the province, he said. They had taken part in all the SDF offensives and were now fighting in Raqqa.

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Nearly 800 fighters from Deir al-Zor’s tribes said on Thursday they had defected from the Syrian Elite Forces, an Arab group fighting alongside the SDF in Raqqa, to join Khawla’s council.

The Syrian army is advancing along the south and west bank of the Euphrates towards Deir al-Zor city. The SDF is mostly on the river’s north and east bank, where Raqqa lies.

Damascus has shored up its rule over much of the country’s populated west with the help of Russian air power and Iran-backed militias.

Now it is marching east towards Deir al-Zor and the vast desert bordering Iraq.

That advance has on occasion brought its forces and allies into conflict with the U.S. military and the groups it backs.

But the rival campaigns have mostly stayed out of each other’s way, and the U.S.-led coalition has stressed it is not seeking war with Damascus.

Islamic State controls most of Deir al-Zor province, and has besieged the government-held pocket of the provincial capital city for years.

In addition to the United Nations, the Syrian government and its Russian ally have made aid drops into the encircled zone, where residents lack food and medicine.

U.N. Syria humanitarian adviser Jan Egeland said there was concern for the civilians in the enclave and others in Islamic State territory as military offensives approach Deir al-Zor.

“We’re also concerned also for our lifeline to the people inside Deir al-Zor (city), some 90,000 people and they only have our air drops,” he said in Geneva on Thursday.