The truce "is still in effect, but it is in great trouble if we don't act quickly," the United Nations' top envoy for Syria Staffan de Mistura told reporters.
A ceasefire took effect in Syria at the end of February, but the country has been rocked by fighting in recent weeks, particularly around the city of Aleppo.
Frustrated by the surging violence and lacking access for desperately needed humanitarian aid on the ground, Syria's main opposition High Negotiations Committee (HNC) earlier this week halted its formal participation in peace talks at United Nations headquarters in Geneva.
"We need to try until Wednesday to get as deep as possible in the areas that we have been starting discussing, and we can do that both formally, informally, technically, practically, but we need to do it," he said.
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De Mistura called on the international community to come together to help strengthen the ceasefire and support the difficult peace drive.
"We do need certainly a new ISSG at the ministerial level, because the level of danger ... (means such a meeting) is urgently required," he said.
US President Barack Obama today voiced alarm at the situation in Syria.
"I am deeply concerned about the cessation of hostilities fraying and whether it's sustainable," he said at a press conference in London with Prime Minister David Cameron.
Syria's conflict erupted in March 2011 with anti-government protests, but has since spiralled into a multi-front war that has left more than 270,000 people dead.