US-backed fighters seek to oust IS from Syria's Deir az-Zour
The upcoming offensive was announced by the Deir az-Zour Military Council [DEMC], a coalition of Arab tribes and fighters that belongs to the broader US-backed Syrian Democratic Forces on Friday,
"Our forces are preparing for the great battle of Deir az-Zour and unifying the tribes," said DEMC head Ahmad Abu Khawlah in Shadai, some 60 kilometres (35 miles) south of Hasakeh.
He said at least 1,500 tribal fighters had joined the DEMC.
"There is no specific timeframe for the battle, but it will be very soon," he said.
SDF fighters have seized some territory in Deir az-Zour province, but Russian-backed Syrian government forces have been making a dash towards the provincial capital of the same name.
The strategic territory is also seen as a prize by advancing Syrian troops, but an agreement between regime ally Russia and the US-led coalition is expected to keep the rival assaults from clashing.
Regime troops have swept across Syria's desert to break IS' two-year siege on tens of thousands of people trapped in Deir az-Zour city.
The US-led coalition has said that a "de-confliction line" has been set to prevent any "mishaps" between the two advancing forces.
Spokesman Colonel Ryan Dillon told journalists this month that the line "has been coordinated and de-conflicted, and has been placed between the SDF and the regime and the Russians and the US".
Abu Muhammad al-Shayti, who heads the DEMC's Shaytat tribal unit, said on Friday that his fighters "will not target Deir az-Zour city, but the province".
"Our forces are committed to what the coalition sees as fit - to fight Daesh (IS) only," he said, using an Arabic acronym for the militant group.
IS has lost swathes of territory to US-backed forces in the north and to Russian-backed Syrian troops in the country's centre and east.
Moscow has said the recapture of Deir az-Zour could mark the conclusion of the battle against IS.
On Wednesday, the US-led coalition's deputy head, British Major General Rupert Jones, said the final fight would probably take place in the stretch of border between Syria and Iraq.
"The expectation has always been that that would see Daesh increasingly squeezed into... the middle Euphrates valley, and that is where the military defeat will be completed," Jones said.
The SDF is currently waging a ferocious fight for IS' de facto capital in Raqqa city, about 75 kilometres (50 miles) west of the administrative border with Deir az-Zour.
Agencies contributed to this report.