Russia hoped Tuesday a new ceasefire could be announced within hours for Syria's battered city of Aleppo, where fresh fighting including rocket fire on a maternity hospital left close to 30 dead.
As the city was struck by some of its heaviest reported clashes in days, Russian Foreign Minister Sergei Lavrov said efforts were under way to agree on a freeze in the fighting.
"I am hoping that in the near future, maybe even in the next few hours, such a decision will be announced," Lavrov said after meeting UN Syria envoy Staffan de Mistura in Moscow.
The UN Security Council will meet Wednesday to discuss the crisis in Aleppo that threatens to derail international peace efforts to end the five-year war in Syria.
France and Britain called for the meeting, boosting the major push by world powers this week to end the fighting.
"Aleppo is burning and it is crucial that we focus on this top priority issue," said Britain's UN ambassador Matthew Rycroft.
A February 27 truce between President Bashar al-Assad's regime and non-jihadist rebels raised hopes for efforts to resolve the five-year conflict.
But it has all but collapsed amid renewed fighting, especially in Aleppo.
A surge of violence that erupted on April 22 has killed more than 270 people in the divided northern city and undermined prospects for peace talks.
In Washington, US Secretary of State John Kerry warned Assad of "repercussions" if his regime continues to flout a new ceasefire being negotiated for Aleppo.
"If Assad does not adhere to this, there will clearly be repercussions and one of them may be the total destruction of the ceasefire and they go back to war," he told reporters.
"I don't think that Russia wants that. I don't think Assad is going to benefit from that."
- UN condemns attacks -
After a relative lull Monday and early Tuesday, rebels in eastern Aleppo fired at least 65 rockets into government-controlled neighbourhoods, said Syrian state news agency SANA.
The rockets killed 16 people and wounded 68, including at least three women at Al-Dabbeet maternity hospital, it reported.
The Syrian Observatory for Human Rights monitor, however, said it had counted at least 19 dead and 80 wounded from the attacks on government-held areas.
Fierce fighting also raged on Aleppo's western edges after rebels detonated explosives in a tunnel, an AFP correspondent said, adding the clashes subsided at nightfall.
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It was the most violent day for the city's regime-held west since clashes resumed 11 days ago.
New regime strikes also hit rebel-held eastern areas.
As warplanes thundered above, rebel and government forces exchanged nearly non-stop artillery fire.
Civil defence workers said air strikes on the rebel-held east killed 11 civilians, including a child.
The rocket attack was the sixth time a medical facility has been hit in 11 days in Aleppo, the International Committee for the Red Cross said, calling it "unacceptable."
The Security Council unanimously voted Tuesday to condemn the targeting of health facilities in war zones.
- 'Regime of silence' -
In Moscow, de Mistura said it was crucial for the ceasefire to be "brought back on track," hailing the February truce as a "remarkable achievement."
Diplomatic efforts were set to continue Wednesday in Berlin, with de Mistura joining the German and French foreign ministers for talks with Syria's main opposition leader.
Washington and Moscow are working together to include Aleppo in a so-called "regime of silence" -- a freeze in fighting -- aimed at bolstering the broader truce.
They have agreed to boost the number of Geneva-based truce monitors to track violations "24 hours a day, seven days a week," Kerry said Monday.
In a nod to Moscow's demands, Kerry said Washington would press moderate rebels to separate themselves from Al-Nusra Front jihadists in Aleppo.
Russia and Assad's regime have cited the presence of Al-Nusra, an affiliate of Al-Qaeda that was not party to the ceasefire, as justifying their offensive.
The Observatory says more than 270 civilians -- including 54 children -- have been killed on both sides of divided Aleppo since April 22.
The city was initially excluded from a deal announced last week to "freeze" fighting along two major fronts in the northwest and in Eastern Ghouta near Damascus.
Meanwhile, the Organisation for the Prohibition of Chemical Weapons said there were "extremely worrying" signs IS may be making its own chemical weapons and may have used them already in Syria and Iraq.
Syria's conflict erupted in 2011 after anti-government protests were put down, and escalated into a multi-faceted war that has killed more than 270,000 people and forced millions from their homes.