Thousands of civilians are trapped in “desperate” humanitarian conditions in a Palestinian refugee camp in Damascus where fighting has been raging for days between Islamic State fighters and other extremists, the UN has warned.

UNRWA, the UN relief agency for Palestinian refugees, said at the weekend that up to 10,000 residents of the Yarmouk camp in the south of the Syrian capital have gone without food or water for more than a week.

“Civilians in Yarmouk are facing starvation and dehydration alongside the heightened risks of serious injury and death from the armed conflict,” said Christopher Gunness, a UNRWA spokesman. People are trapped in their homes, hunkered down to avoid being hit by bullets and shrapnel, he added.

The camp, a sprawling urban neighbourhood that was once home to 150,000 people, has been ravaged by fighting between Isis and al-Qaida’s Syrian affiliate, al-Nusra Front, while government forces regularly shell it from outside. “Whatever supplies of food and water they had have long been exhausted,” Gunness said.

Humanitarian missions are on standby to deliver aid to Yarmouk as soon as the situation allows such access, the UNRWA said.

The plight of civilians across Syria will be on the agenda again for talks resuming on Monday in Geneva between the UN envoy, Staffan de Mistura, representatives of Syrian President Bashar al-Assad’s government and the rebels fighting to overthrow him. A second round began at the end of last week with no sign of any narrowing of the gap between the two Syrian sides, who have not yet met face to face.

Aid deliveries to besieged opposition areas are still being blocked by the Syrian government, the UN has said, amid mounting evidence that the authorities in Damascus are adamant about not discussing the political transition that De Mistura has called “the mother of all issues”. Assad’s future has been described as a “red line” by the regime.

On Sunday Riyad Hijab, the head of the opposition Higher Negotiations Committee (HNC), met diplomats from the US, Britain, France, Saudi Arabia, Qatar and Turkey to plan for the next stage of talks. Last Friday Bashar Jaafari, the chief Syrian government negotiator, again sought to move away from discussion of transition by handing De Mistura amendments to a 12-point plan issued by the UN in March.

“It is the responsibility of Mr De Mistura to make sure that everyone sticks to the agenda,” said Salem al-Muslet, an HNC spokesman. “If the regime does not want to discuss the transitional governing body then they are wasting time.”

The Syrian National Coalition, another mainstream opposition group, hinted at withdrawal from the Geneva talks, saying it “does not rule out any option should the Assad regime carry on with its crimes and the UN special envoy persist with his lack of commitment to the mission entrusted to him by the United Nations”.

Analysts believe that the Syrian government, flatly refusing to make any concessions, may be testing whether it can sabotage these talks by escalating violence, a strategy which may succeed if there is no punitive international response.

On Saturday, a powerful Islamist rebel group warned that the Geneva process was divorced from the deteriorating military situation on the ground, which is threatening a patchy six-week truce agreement brokered by the US and Russia.

Ahrar al-Sham also said there was “a gulf” between the HNC negotiating body and “the revolutionary street with all its military and civilian elements”, calling its performance “weak and stumbling” while Russia and Iran were helping the government make territorial gains.

The Ahrar statement also noted that important opposition conditions for the start of the political process had still not been realised, including an end to government blockades of opposition-held territory as well as the release of thousands of detainees. “We are here for a political solution,” Muslet said. “They can say what they want but we have our principles and we will stick to them.”