Speaking after briefing the Security Council on the peace process, Staffan de Mistura said a partial truce agreed in February was "barely alive".
Violence in Syria has intensified in recent days, despite the ceasefire.
At least 20 civilians were reportedly killed on Wednesday in government strikes on a hospital and nearby residential building in eastern Aleppo.
The dead included children, a dentist and the only paediatrician left in rebel-held areas of the city, civil defence volunteers told AFP news agency.
The upsurge in fighting in Syria's largest city comes amid reports that government forces, backed by Russian air power, are gearing up for a major offensive there.
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The escalation has threatened to derail the UN-brokered peace talks, which resumed last month.
The Western-backed opposition delegation, the High Negotiations Committee (HNC), last week suspended its role to protest against alleged ceasefire violations by the government and a fall in humanitarian aid deliveries to besieged areas.
Speaking on Wednesday after a third round of talks in Geneva, Mr de Mistura said the fragile "cessation of hostilities" "could collapse any time".
He said that over the past 48 hours an average of one Syrian had been killed every 25 minutes and one wounded every 13 minutes.
For the peace talks to succeed, Mr de Mistura said, hostilities would need to be reduced to the levels seen immediately following the February agreement.
Calling on the US and Russia to co-operate, Mr de Mistura said that the legacies of both President Barack Obama and President Vladimir Putin were linked to the success of the peace process in Syria.
There will be one or two more rounds of talks before July, Mr de Mistura said. He added: "There are still major differences on the major issues, but there is movement on certain areas where there was not before."
Mr de Mistura also said that equal rights and equal representation in major institutions for women was essential to the transition to a new Syria.
The peace talks do not involve the al-Qaeda-linked al-Nusra front and the so-called Islamic State, which have been fighting government forces and other rebels across Syria.
The UK-based Syrian Observatory for Human Rights monitoring group confirmed on Wednesday that the air strikes on a hospital and nearby buildings in Aleppo were carried out by "regime airplanes".
Separately, 11 people were killed in western Aleppo earlier on Wednesday, the Syrian Observatory said.
More than 270,000 people have been killed since Syria's bitter civil war conflict erupted in 2011 and millions have been forced to flee.
The UN envoy to Syria has urged the US and Russia to intervene "at the highest level" to save struggling peace talks.