Islamic State militants have captured part of the last remaining government enclave in eastern Syria, a monitoring group said on Wednesday, as peace talks stumbled in Geneva without the full participation of the opposition.

The Syrian Observatory for Human Rights said Islamic State had seized the industrial district of al-Sanaa in the eastern city of Deir al-Zour from Syrian government forces late Tuesday.

The extremists were advancing towards a regime-held military airport in the area, the Britain-based group added.

Islamic State controls almost all of Deir al-Zour province, but much of the city is still in the hands of the Syrian government.

The al-Qaeda splinter group has since 2014 besieged the regime-held areas, where an estimated 200,000 people live.

Syrian state media said government forces withstood an Islamic State attack on Deir al-Zour military airport and lured the jihadists into a trap in the al-Sanaa area.

Last month, the Syrian Army, backed by Russian airstrikes, drove Islamic State out of the ancient city of Palmyra in central Syria.

The country's fragile ceasefire, mediated by the United States and Russia, excludes Islamic State and al-Nusra Front.

On Wednesday, regime forces stepped up their air bombardment of areas in northern and north-western Syria controlled by the al-Qaeda-linked al-Nusra Front and Western-backed rebels, the observatory said.

The attacks came after at least 46 people, including civilians, were killed Tuesday in regime strikes in Idlib, according to the watchdog.

The opposition has condemned the latest surge of violence and decided to suspend its participation in UN-sponsored peace talks in Geneva.

"With the massacres committed by the regime yesterday [Tuesday], the regime has buried the ceasefire," said Anas al-Abdeh, the head of the main opposition bloc, the Syrian National Coalition, referring to a truce that started in February.

Al-Abdeh said the opposition will not return to negotiations unless the violence stops.

"If there aren't any changes, negotiations could break down completely," he said at a press conference in Istanbul.

In Geneva, Syria's chief government delegate, Bashar Jaafari, met with UN mediators and said afterwards that the talks were better off without the opposition.

The opposition was made up of "extremists, terrorists and mercenaries" who did not represent Syria as they were controlled by Saudia Arabia and Turkey, Jaafari charged.

"By leaving, they may be taking away a major obstacle, and that will allow us to reach a solution," he said, without making clear how he would continue talks without counterparts.

While some opposition delegates have left Geneva, some have stayed behind to continue unofficial talks with UN peace-brokers.

Meanwhile, the UN said it was evacuating some 500 people from besieged areas with the help of humanitarian organization, the Syrian Arab Red Crescent.

"Today [Wednesday] plans are under way to evacuate some 500 people, including the sick, wounded and their family members, from the besieged four towns - which are Foua, Kefraya, Madaya and Zabadani - in urgent need of life-saving medical attention," UN spokesman Stephane Dujarric said.

Most evacuations are being done by road, he said, noting that there's no timeframe for when the operations will be finished.

Some 250 people were due to be evacuated from rebel-held Madaya and Zabadani, near the Lebanese border, which are besieged by government forces.

The same number were to be transported from the besieged government-held villages of Foua and Kafraya in northern Syria.

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