A final assault on Islamic State's last line of defense in its former Syrian capital Raqqa should begin on Sunday night, a field commander for the U.S.-backed forces operating there said.
The loss of Islamic State's remaining streets and buildings in Raqqa following its defeat in Iraq's Mosul this year and its retreat from swathes of territory in both countries, would mark a big moment in the battle to destroy the jihadist group.
At the height of its power two years ago Islamic State ruled over millions of people, from northern Syria to the outskirts of Iraq's capital Baghdad, but it has since endured a series of losses under attack from many sides.
The assault on militants in the center of Raqqa will focus on the area around the stadium and would attempt to surround it, Ardal Raqqa, a field commander in the Syrian Democratic Forces SDF) in western Raqqa, told Reuters on Sunday.
For three years Raqqa was the de facto Syrian capital of Islamic State's self-declared caliphate, a center of operations where it oversaw the management of much of eastern, central and northern Syria and planned attacks abroad.
Now it is hemmed into a small area in the city center that includes the stadium, the National Hospital and a roundabout where Islamic State once displayed the heads of its enemies.
The group has lost most of its territory to the SDF and to a rival offensive by Syria's army and allied forces this year, and has fallen back on the fertile Euphrates valley area downstream of Raqqa.
The army and its allies reached the city of Deir al-Zor in September after a months-long offensive across the Syria desert, and have since then pushed down the Euphrates towards the border with Iraq.
On Sunday a Syrian military source said they had encircled Islamic State fighters in the city of al-Mayadin, one of the jihadists' last strongholds in the area.
"Units of our armed forces with the allied forces continue their advance on a number of fronts and axes in Deir al-Zor and its countryside... and encircle Daesh terrorists in the city of al-Mayadin," the military source said.
However, the group has still been able to launch a series of effective counter attacks against the Syrian army in the central desert region over the past week, putting pressure on the main supply road to Deir al-Zor from the west.
Syrian President Bashar al-Assad is backed in the war by Russia, Iran and Shi'ite militias including Lebanon's Hezbollah, and its campaign against Islamic State has mostly been on the west bank of the river.
The U.S.-backed SDF campaign has mostly been on the east bank, where Raqqa is located, and has also advanced downstream to hold areas opposite Deir al-Zor. The U.S. and Russia have put in place channels to lessen the risk of fighting between the rival offensives they back.
U.S. officials have previously said that Islamic State had relocated some of its diminished command and propaganda structures to al-Mayadin as it was forced from territory elsewhere.
The spokeswoman for the SDF campaign in Raqqa, Jihan Sheikh Ahmad, said in a statement on a website for the campaign that it would announce the liberation of Raqqa "in the coming few days" after having captured 85 percent of the city.
Commanders directing the battle in Raqqa have said that Islamic State fighters have taken civilian hostages and are using sniper fire, booby traps and tunnels to slow the SDF advance.
The SDF, spearheaded by the Kurdish YPG militia, began its campaign to isolate Raqqa early this year, pushing along several fronts to enclose the city against the Euphrates backed by coalition air strikes and special forces.
Its attack on the city itself started in June and the fighting left much of Raqqa in ruins, as intense air strikes and street-to-street battles devastated buildings.