US-backed fighters launched a new offensive on Saturday to oust the Islamic State (IS) militants from swathes of Deir Ezzor, the last Syrian province still largely under Islamist militant control.
The oil-rich province borders Iraq and is a strategic prize for many, including the Russian-backed Syrian troops that are battling for its western half.
The new assault comes with IS under growing pressure in Syria and neighbouring Iraq after a string of defeats.
IS have lost more than half of their de facto Syrian capital Raqa and all of Iraq's second city, Mosul.
On Saturday, the US-backed Syrian Democratic Forces (SDF) announced they had begun fighting to clear IS group from territory east of the Euphrates River, which runs diagonally across Deir Ezzor province,cutting it in half.
"We are taking the first step to liberate territory east of the Euphrates River in Deir Ezzor," Ahmad Abu Khawlah, who heads the SDF's Deir Ezzor Military Council (DEMC), told AFP.
"Pivoting towards Deir Ezzor was inevitable."
Abu Khawlah said US-led coalition air strikes had already helped his forces advance "around 30 kilometres (20 miles)" in the first day of "Operation Jazira Storm."
Military operations on Saturday were focused in the hilly desert area in northeastern parts of Deir Ezzor province, according to the Syrian Observatory for Human Rights, a British-based monitor.
"Clashes are ongoing and coalition air strikes are hitting jihadist positions to allow the SDF to advance on the ground," said Observatory head Rami Abdel Rahman.
Since 20(DEMC)14, IS has held swathes of the eastern province and about 60 percent of the provincial capital, Deir Ezzor city.
The coalition has bombed IS targets in Syria and Iraq since 2014 and is now backing the SDF's offensive to capture Raqa.
The SDF has seized around 65 percent of Raqa city, which lies directly west of Deir Ezzor province.
"Daesh will have no safe haven in the Euphrates River Valley," coalition spokesman Colonel Ryan Dillon said, using the Arabic acronym for IS.
"We continue to support our SDF partners as they fight to liberate their lands and homes from IS," Dillon told AFP.
Saturday's announcement came as Russia-backed Syrian troops wage a separate offensive to oust IS from the west of Deir Ezzor province, as well as the provincial capital.
Abu Khawlah said SDF forces were not coordinating with the government or with Russia on military operations.
But the coalition, the SDF, Syria's government and Russia have agreed on a "de-confliction line" in northeast Syria to prevent them clashing.
According to the coalition, the line runs from Raqa province southeast along the Euphrates River towards Deir Ezzor.
"The de-confliction line with the Russians is necessary in the complex and congested battlespace over eastern Syria," Dillon said.
On Saturday, Syrian news agency SANA reported that regime troops captured the Al-Taym oil field south of Deir Ezzor's key military airport, which is held by the government but surrounded by IS.
Regime loyalists launched an attack to break IS's siege on the airport on Friday, as part of their multi-pronged offensive to capture the whole city from IS.
The foreign ministry in Damascus on Saturday hailed the army advance, which it said "foreshadowed the end of terrorism."
"The battle with these mercenaries is nearing its end, so that every inch of Syrian land will be free of terrorism," it said in a statement distributed by SANA.
Syria's conflict erupted in 2011 with protests demanding that President Bashar al-Assad step down, but it has since evolved into a complex, multi-front war.
More than 330,000 people have been killed and millions have fled their homes, but global efforts at reaching a peace deal have faltered.
Next week, Kazakhstan will host talks on a set of safe zones across Syrian territory agreed by world powers this year as a first step towards a political solution.
And the United Nations is hoping to gather members of the Syrian opposition and the government for negotiations in Geneva in October.