US-backed militias and the Syrian army are advancing in separate offensives against IS in eastern Syria, piling pressure on shrinking territory the group still holds in oil-rich areas near the Iraqi border.
Syrian government forces fought their way to an air base on the outskirts of Deir al-Zor city that had been besieged for years by the jihadists, said a commander in the military alliance fighting in support of President Bashar al-Assad.
The Syrian Democratic Forces, a US-backed alliance of mostly Arab and Kurdish fighters, meanwhile launched attacks against IS in the north of Deir al-Zor province in an operation to capture areas east of the Euphrates river.
The advances against IS, another blow to its control over territory it held for years as part of a self-declared caliphate, will likely bring US-backed forces and the Syrian government side, backed by Russia and Iran, into closer proximity.
The SDF operation in Deir al-Zor province aims to capture areas in its northern and eastern countryside and advance towards the Euphrates, according to the Deir al-Zor Military Council, which is fighting as part of the SDF.
"The first step is to free the eastern bank of the Euphrates and the areas Islamic State still holds," Ahmed Abu Kholeh, head of the military council, told Reuters after the announcement.
"We're not specifying a time frame but we hope it will be a quick operation," he said at the town of al-Shadadi in Hasaka province.
Abu Kholeh would not say whether there were plans to advance on Deir al-Zor city itself. "We don't know how the battles will go after this," he said.
He said SDF fighters did not expect clashes with Syrian government forces, but if fired upon "we will respond".
The Syrian Observatory for Human Rights monitoring group reported that SDF forces had advanced against IS in Deir al-Zor's northwestern countryside, seizing several hilltops and a village.
Syrian government forces and their allies reached Deir al-Zor military airport on the other side of the Euphrates, where troops had been holed up since 2014, surrounded by Islamic State, the commander in the pro-Assad alliance said.
The advance came days after the army and its allies broke the siege of the main part of the city, which had been separated from the airport by IS attacks a few months before.
Syrian troops also recaptured the Teym oilfield southwest of Deir al-Zor and seized part of a main highway running downstream to the city of al-Mayadeen, to which many IS militants have retreated, the British-based Observatory said.
