The U.S.-backed Syrian Democratic Forces (SDF), which is predominantly led by the PKK's Syrian affiliate, the People's Protection Units (YPG), will soon launch an offensive against Daesh in Syria's Deir ez-Zor province, an official from the alliance told Reuters Friday.

Ahmed Abu Khawla, head of the Deir ez-Zor military council operating under the SDF, said the assault might begin "within several weeks" parallel to the battle for Raqqa city.

Backed by the U.S., the SDF started battling inside the historic Old City of Raqqa, Daesh's de facto capital in Syria, in June, after months of fighting to encircle Raqqa with air strikes and special forces from the U.S.-led coalition.

As Daesh comes under pressure in Raqqa, many of its forces have fallen back on the towns and cities further east along the Euphrates River in Deir ez-Zor province.

"The operation to liberate Deir ez-Zor will begin very, very soon," Khawla said, adding that his unit had already "entered Deir ez-Zor territory and liberated several towns" there.

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

Khawla said there were 4,000 SDF fighters in the Deir ez-Zor military council that he heads.

They had taken part in every SDF campaign and were now fighting inside Raqqa, he said.

U.S. support of the SDF, led by the YPG, and partnership in the operation has become a matter of dispute with Turkey, which says one terrorist group cannot be eradicated by another. The YPG's attempts to change regions' demographics captured from Daesh will lead to further regional turmoil, as well as oppression of Arab and Turkmen locals.

Syria's government, backed by Russia, Iran and Shiite militias, is engaged in its own campaign in eastern Syria aimed at relieving its besieged enclave in Deir ez-Zor.

The Syrian army is advancing along the south and west bank of the Euphrates toward where the city of Deir ez-Zor is located, while the SDF is mostly on the north and east bank, where Raqqa is located.