The prospects of a negotiated peace settlement in the Syrian civil war have been dealt a serious blow as the opposition’s chief negotiator in UN-brokered talks in Geneva resigned, describing them as a waste of time on security and humanitarian fronts.

Mohammed Alloush, a member of the Saudi-backed rebel group Jaysh al-Islam (Army of Islam), condemned the international community’s “inability to enforce resolutions, in particular regarding humanitarian issues, [such as] the lifting of sieges, access to aid, the release of prisoners and adherence to the ceasefire”.

Alloush, who had been under pressure from his own military supporters to resign, said in a statement on Facebook on Sunday: “The three rounds of talks were unsuccessful because of the stubbornness of the regime and its continued bombardments and aggression towards the Syrian people.”

Before his resignation, Alloush had called on Russia and the US to do more to enforce a ceasefire. He had also called for more action on the release of political prisoners and their treatment while incarcerated.

Last week the UN’s envoy, Staffan de Mistura, postponed talks for three weeks, saying more time was needed to re-establish the cessation of hostilities and humanitarian access to besieged cities.

Previous rounds of talks in Geneva made little progress on the central issue of a political transition. Many on the opposition High Negotiations Committee (HNC) believe the US is focusing on removing Isis from its de facto capital, Raqqa, rather than the battle to dislodge Bashar al-Assad.

Asaad al-Zoubi, the head of the main Syrian opposition delegation, also said he wanted to be relieved of his post in the HNC, but did not confirm he had taken a similar step.

The HNC said Alloush’s decision would lead to a wider restructuring of its leadership. The move also reflects tensions about whether it should pull out of the talks altogether and in effect disband.

Alloush’s importance to the HNC stemmed from his closer links to Syrian fighters. Some members of the committee – an unwieldy group assembled in Riyadh – were regarded as exiles who have less direct contact to those fighting Assad’s forces. The danger for those trying to keep the peace process alive is that the HNC will lose legitimacy if it does not have sufficient links with rebel fighters.

The Syrian government’s chief negotiator in Geneva, Bashar Jaafari, had described Alloush as a terrorist, refusing to talk to him unless he “shaves off his beard”. Forces linked to Alloush have been accused of atrocities in Damascus suburbs.

The UK special envoy to Syria, Gareth Bayley, said on Monday that Britain was ready to help to deliver aid to starving towns using airdrops if the Assad government did not allow access to besieged towns by Wednesday.

There have been fears that the international community will backtrack on a commitment to use airdrops as a last resort from a 1 June deadline set by the International Syria Support Group.

Bayley said the Assad government was still using starvation as a weapon of war. “Airdrops to deliver aid to designated besieged areas remains a last resort,” he said in a statement. “It is an expensive and complex way to deliver aid, but its vital that we fulfil this commitment. The UK stands ready to do so.”

It is unlikely that the UK would act independent of the UN or the World Food Programme, but the remarks will put pressure on the UN and the United States not to backtrack on its commitment.