BEIRUT: The Syrian opposition's chief negotiator in UN-brokered peace talks has announced his resignation, branding the stalled Geneva negotiations a failure on both the security and humanitarian fronts.

"The three rounds of talks were unsuccessful because of the stubbornness of the regime and its continued bombardments and aggressions towards the Syrian people," Mohammed Alloush, a member of the Saudi-backed rebel group Jaish al-Islam (Army of Islam), said late Sunday in a statement on Twitter.

He also 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".

The regime of President Bashar al-Assad and rebels agreed a shaky ceasefire in February to bolster the peace talks but repeated violations have left the truce hanging by a thread.

"The endless negotiations are harming the fate of the Syrian people," Alloush added.

"I therefore announce my withdrawal from the delegation and my resignation" from the main opposition High Negotiations Committee (HNC).

The last round of peace talks in Geneva reached a deadlock in April when the HNC suspended its participation over an escalation of fighting on the ground.

A new round of talks had been expected for the end of May, but no new date has been announced.

The UN's peace envoy for Syria, Staffan de Mistura, said on Thursday that he had no plans to convene another round in the next two or three weeks.

Expected 'for some time'

Diplomats have said there was little chance that the opposition would take part in new peace talks if violence was raging and no aid was reaching civilians.

The talks aim to reach a political settlement to end the five-year war that has left 280,000 dead and driven millions from their homes.

The fate of Assad has been a key stumbling block in the negotiations, with the opposition insisting any peace deal must include his departure while Damascus says his future is non-negotiable.

With little high-level negotiations experience, Alloush was a controversial choice as the HNC's chief negotiator.

Opposition members have also criticised Jaish al-Islam for its alleged involvement in kidnapping prominent rights activists in the town of Douma.

Syria analyst Charles Lister said Alloush's resignation had been "discussed for some time" and may trigger additional defections, including HNC delegation head Asaad al-Zoabi.

Zoabi defected from the Syrian air force in August 2012 and began advising rebel groups in Syria's south from Jordan.

The prominent roles that both Alloush and Zoabi held within the HNC have boosted the body's legitimacy among rebels on the ground, who have previously derided the opposition-in-exile as nothing more than suits in hotels.

On the ground in Syria, meanwhile, thousands of civilians fled a fresh offensive by the militant Islamic State group (IS) in the north, the Syrian Observatory for Human Rights said Sunday.

The surprise offensive against the towns of Marea and Azaz threatens to overrun the last swathe of territory in the east of Aleppo province held by rebels.

Further east, US-led coalition warplanes targeted IS positions north of the IS bastion of Raqqa, killing 45 IS fighters, the Observatory said.