The deteriorating situation coincides with increased tension between the two drivers of the truce, Russia and the United States, which accuse each other of derailing the ceasefire.
On Saturday the situation tightened further after the US-led coalition bombing in the east positions, an ally of Moscow Syrian army, killing at least 90 troops, according to the latest report of the Syrian Observatory for Human Rights ( OSDH).
Russia on Sunday called on Washington a full investigation into the attack. "Moscow is deeply troubled by what happened. We ask our American partners to carry out the most thorough investigation and take measures to prevent such incidents in the future," he said Russian foreign ministry said in a statement.
The coalition acknowledged that it had bombed what he thought was a position of the jihadist group Islamic State (EI) and the operation ended when Moscow warned him that it was perhaps Syrian military.
Required by Moscow, the Security Council of the UN met Saturday night with urgency.
The Minister of Syrian President Bashar al-Assad, Buthaina Chaban, accused the coalition of launching a deliberate attack.
"Everything was premeditated. Dáesh (Arabic acronym for the jihadist group EI) was aware as advanced shelling ceased," he told AFP by telephone from Damascus.
This incident is "a bad omen" for the Russian-American agreement remains in Syria, he said on his part to press the Russian Ambassador Vitali Shurkin.
Meanwhile, the head of French diplomacy, Jean-Marc Ayrault said that the primary responsibility for violating the truce in Syria was the Syrian regime.
Sunday regime forces launched an offensive in Deir Ezor to try to reconquer the positions taken yesterday by the EI. A Syrian military source reported that the EI army shot down a plane and the pilot died.
Substantially all of the province of Deir Ezor is in the hands of EI except military airport and several adjacent neighborhoods controlled by the regime.
First airstrike on Aleppo
The OSDH reported Sunday the rebel zone of Aleppo suffered four air raids, the first since the start of the truce.
In this city of northern Syria, economic ex-capital the country, is the main front in a war that has left more than 300,000 dead since 2011.
The US-Russian agreement truce provided, inter alia, sending humanitarian aid to rebel-held areas of the city where they live, according to the UN, about 250,000 people, besieged by the forces of President Bashar al-Assad.
In the absence of sufficient guarantees trucks of food and medicine they were still blocked at the border between Turkey and Syria.
According to the OSDH a total of 10 civilians were killed Sunday in the bloodiest day since the start of the truce.
In recent days, despite the truce, clashes multiplied in the Eastern Guta, east of the capital, in the northern province of Hama and Homs, in the center of the country and in the west, in the mountainous province of Latakia.