UN Special Envoy Staffan de Mistura voiced deep concern at the truce unravelling in Aleppo and at least three other hotspots.

However, he said he saw some narrowing of positions between the government and opposition visions of political transition.

Mr de Mistura told a news conference that "the legacy of both President [Barack] Obama and President [Vladimir] Putin is linked to the success of what has been a unique initiative which started very well."

He added: "It needs to end very well."

Twenty people died after an air strike hit a hospital in a rebel-held area of Aleppo, the Syrian Observatory for Human Rights said today.

Three children and the last paediatrician in the city were among the dead.

The Civil Defence rescue service in rebel-held areas of Aleppo put the death toll at 30.

The observatory said 84 civilians had been killed in government air strikes in Aleppo and 49 civilians were killed in rebel shelling of government-held areas in the past six days.

Mr de Mistura said the US and Russia had a common interest in not seeing Syria ending up in another cycle of war.

He said the US and Russia must convene a ministerial meeting of major and regional powers who compose the International Syria Support Group.

The main opposition High Negotiations Committee pulled out of the formal talks a week ago, in protest of intensified fighting and slow aid deliveries.

Mr de Mistura said he aimed to resume talks in May, though he gave no date.

Bashar Ja'afari, who led the government delegation, said this week the round had been "useful and constructive".

But he gave no sign of ceding to the HNC's central demand for a political transition without President Bashar al-Assad.

Asked whether Mr Assad's future was discussed, Mr de Mistura said: "We didn't get into names of people ... but actually how to change the current governance.

"And I must say that the concept of a new government and a political transition with a new constitution is quite a lot already in terms of preparing of what could be the next steps."

In the past 48 hours there had been an average of one Syrian civilian killed every 25 minutes and one wounded every 13 minutes, he said.

The latest victims included "the last paediatric doctor" in rebel-held eastern Aleppo, killed in an air strike on a hospital yesterday.