Several major Syrian rebel factions vowed Monday 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,'" a statement from 10 rebel factions said.

Among the groups were Islamist factions Army of Islam and Ahrar al Sham, two of the most powerful rebel forces in the country.

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.

Aleppo is divided between government and rebel control, with President Bashar al-Assad's forces almost entirely encircling the opposition-held areas.

The rebels are concerned about a potential government siege on their eastern portion with only one main road currently open to them.

Mohammed Alloush, a high-ranking official of the Jaish al-Islam rebel group, who is also the chief negotiator for the opposition in the UN-backed peace talks taking place in Geneva, called on Sunday for retaliation against the government.

"Don't trust the regime. Hit them at their necks ... strike them everywhere," Alloush wrote on his Twitter account from the Swiss city.

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

Key members of the Syrian opposition have said al-Assad cannot be part of any future transitional government in the country.

Meanwhile, the rebels, including Ahrar al Sham and the al-Qaeda wing al-Nusra Front, are also 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, as clashes are ongoing between Islamic State and rebels, aid group Doctors Without Borders (MSF) warned.

Over the past 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 ongoing conflict.

The Turkish border remains largely shut, with some exceptions being made for medical cases. Some clinics in the Syrian city of Azaz at the Turkish border have been forced to shut, while those still open are under intense pressure, flooded with cases.

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

Two people were killed and five injured after rockets landed in southern Turkey from Syria, the Dogan news agency reported. Last week, Turkey blamed Islamic State for consecutive days of rocket fire on Kilis province and the army shelled the extremists in retaliation.

Islamic State has managed to reverse recent rebel gains along the border, as efforts are stepped up to oust the extremists from its last territory along the frontier.

Turkey is backing some rebel factions and the rebel setbacks are seen as blow to efforts to make sure Kurdish forces, supported by US airstrikes, do not take the territory.

"Turkey has been trying to organize these guys along their border since 2012. It just hasn't worked out," said Aaron Stein, an analyst with the Atlantic Council.

The US, whose primary objective in Syria is to defeat Islamic State, is unwilling to work with hardline Islamic groups that Turkey tolerates.

MSF was also critical of the European Union, amid the potential humanitarian crisis on the border.

"It is unacceptable that the current efforts of the EU are focused on how to return Syrian refugees to Turkey, instead of on how to assure safety and protection for those amassing at the Syrian-Turkish border," the aid group said.

Turkey and the EU reached a deal last month meant to stem the flow of migration to the bloc.

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