By John Davison and Rodi Said
BEIRUT/AL SHADADI, Syria (Reuters) - U.S.-backed militias and the Syrian army advanced in separate offensives against Islamic State in eastern Syria on Saturday, 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.
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The Syrian Democratic Forces (SDF), a U.S.-backed alliance of mostly Arab and Kurdish fighters, meanwhile launched attacks against Islamic State in the north of Deir al-Zor province in an operation to capture areas east of the Euphrates river.
The advances against Islamic State, another blow to its control over territory it held for years as part of a self-declared caliphate, will likely bring U.S.-backed forces and the Syrian government side, backed by Russia and Iran, into closer proximity.
A U.S. warplane shot down a Syrian army jet near Raqqa in June and the SDF accused the Syrian government of bombing its positions, showing the risk of escalation between warring sides in a crowded battlefield.
The Syrian conflict, which started as a popular uprising against Assad in 2011, has drawn in the United States, Russia and regional powers. Peace talks have failed to bring an end to a war where Islamist groups have increasingly dominated Syria's armed opposition.
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 timeframe 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 ARMY RELIEVES AIRPORT
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 alliance includes Iran-backed militias and the powerful Lebanese Shi'ite group Hezbollah.
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.
That advance would help block potential IS reinforcements from al-Mayadeen, it said.
Islamic State in Syria still holds much of Deir al-Zor province and half the city, as well some territory further west near Homs and Hama, where government forces recaptured several villages on Saturday, pro-Damascus media reported.
But the group has lost most of its caliphate which from 2014 stretched across swathes of Syria and Iraq, including oil-rich Deir al-Zor.
The SDF is still battling to eject IS from the remaining areas it holds in Raqqa, northwest of Deir al-Zor and once the group's main Syria stronghold from where it planned attacks abroad.
Talks between Russia, Iran and opposition backer Turkey in the Kazakh capital Astana are to take place next week, possibly followed by a separate track at the United Nations in Geneva in October or November.
Assad's government has participated in previous rounds from a position of power as Damascus has clawed back much territory, including the main urban centers in the west of the country and increasingly eastern desert held by IS.
Syria's non-Islamist opposition holds some pockets of territory in western Syria, and the SDF, which is dominated by the Kurdish YPG militia, controls much of Syria's northeast.
(Additional reporting by Laila Bassam in Beirut; Editing by Gareth Jones)