Syrian opposition negotiators and rebels temporarily withdrew Monday from UN-brokered peace talks and vowed to strike back in reaction to the government's alleged ceasefire violations.

While an opposition spokesman said his side had "decided to postpone" the talks that started Wednesday in Geneva, UN mediator Staffan de Mistura clarified that the delegation had left an opening.

The delegates would suspend their participation in official meetings at UN offices in the Swiss city, he said.

"They told us, however, their intention to remain in Geneva in their hotel and possibly, at my own suggestion, to pursue technical discussions with myself and my team," de Mistura told reporters.

The opposition decided that continuing negotiations despite a lack of progress on humanitarian issues and escalating ceasefire breaches would be "increasing the ordeal of our people," spokesman Ahmed Ramadan said.

De Mistura has been shuttling between the regime and the opposition teams, rather than bringing them to the same table.

US President Barack Obama telephoned Russian President Vladimir Putin to express concern about the status of the Syrian ceasefire and push Putin to pressure the Russian-backed regime of Syrian President Bashar al-Assad to stop attacks against the opposition.

"The two leaders committed to intensify their efforts to shore up the cessation of hostilities and affirmed the need to end attacks by all parties and ensure humanitarian access to all besieged areas," the White House said.

Earlier Monday, several major Syrian rebel factions vowed to retaliate against government forces for alleged violations of the cessation of hostilities, in a further sign that the war-torn country's ceasefire is teetering.

Rebel troops attacked government positions in Latakia province in the north and on the outskirts of western Hama in Syria's central region, according to the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman.

The latest clashes come on the heels of recent skirmishes and a build-up of forces at potential flashpoints.

"We announce the formation of a joint [military] operations room and the start of the battle called 'Response to Grievances,'" said a statement from 10 rebel factions including the powerful Islamist groups Army of Islam and Ahrar al-Sham.

The opposition accuses the government of using the ceasefire, which was brokered by Russia and the United States on February 27, to gain ground and prepare for future attacks, especially around Aleppo city.

"The cessation of hostilities is still holding in many areas, but the increase in fighting is indeed worrisome," said de Mistura, summing up the United Nation's assessment.

Humanitarian shipments were proceeding too slowly he, he said.

In the Geneva talks, the opposition has been demanding a transitional government without the involvement of President Bashar al-Assad, while the government side has ruled out an early departure of the president.

"On the political track, no-one should expect that after five years of a conflict a political transition by miracle is solved in one week," de Mistura said.

The Swedish-Italian UN diplomat said he would continue the talks at least until Friday, when he plans to take stock and decide on the future schedule.

Mohammed Alloush, a high-ranking official of the Jaish al-Islam rebel group and chief negotiator for the opposition in the peace talks, called Sunday for retaliation.

"Don't trust the regime. Hit them at their necks. ... Strike them everywhere," Alloush wrote on Twitter.

Speaking in Geneva, Syrian UN envoy Bashar al-Jaafari on Monday criticized "irresponsible and provocative statements" from the opposition. He did indicate that the government was interested in continuing indirect talks through the UN mediator.

Meanwhile, rebels including Ahrar al-Sham and the al-Qaeda wing al-Nusra Front are under pressure from the Islamic State extremist militia in the northern province of Aleppo.

More than 100,000 people are trapped in a small pocket in northern Syria, amid ongoing clashes between Islamic State and rebels, the aid group Doctors Without Borders (MSF) warned.

Over the last week, more than 35,000 people have fled camps for displaced people, which were taken over by Islamic State or which are close to front lines, the medical charity said. They join those stranded during previous phases of the conflict.

Turkey's government has said it is allowing aid into Syria to reach these in need.

Three children and adult from Syria were killed after rockets landed in southern Turkey from Syria, the Dogan news agency reported.

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