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.
Russia's continuing war
Has opportunity for peace been lost?
What is left of Syria?
Assad's growing confidence
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.

Leon Mugesera, an academic, was extradited back to his home country from Canada four years ago.
He made his incendiary speech against the Tutsi minority in 1992. Some 800,000 people died in the genocide.
Mugesera later worked as a lecturer in Quebec province and lost a 12-year legal battle to avoid extradition.
In 1992, then an official in Rwanda's ruling Hutu party, Mugesera  told more than 1,000 party members that they should kill Tutsis and dump their bodies in the river.
Mugesera later maintained his innocence, saying the speech had been taken out of context.
The genocide ended when rebels, led by current Tutsi President Paul Kagame, seized power in July 1994.
Militias from the majority Hutu ethnic group were blamed for the mass slaughter.

12 March 2016 Last updated at 08:26 GMT
One of the bears went over to check out the strange object, giving it a quick sniff, before being joined by two others.
The bears batted it around until their sharp claws eventually popped it.
Xaviera Simons was visiting the safari park when she filmed the moment.

Gordon Culliford, 77, from Ammanford, Carmarthenshire, was last seen at about 13.15 BST on Thursday at the city's Singleton Hospital.
He is white, 5ft 7in tall, with white swept back hair and was last seen wearing black glasses and a red jacket.
South Wales Police said there is "considerable concern" for the man and called or information.
The UN envoy to Syria has urged the US and Russia to intervene "at the highest level" to save struggling peace talks.
[NEXT_CONCEPT]
A former politician who described Tutsis as "cockroaches" and called for their extermination has been jailed for life in Rwanda over the 1994 genocide.
[NEXT_CONCEPT]
When a stray pink balloon drifted into the sloth bear enclosure at a zoo in the Netherlands the curious bears there saw it as a great opportunity to play.
[NEXT_CONCEPT]
A search has been launched for a pensioner who has gone missing in Swansea.