Desperate efforts to salvage Syria's ceasefire shifted to Moscow on Tuesday as the country's battered second city of Aleppo came under fresh fire that killed at least four people.
Foreign Minister Sergei Lavrov was due to hold talks with UN envoy Staffan de Mistura in the Russian capital in a last-ditch bid to rescue peace negotiations that have been undermined by fierce fighting around Aleppo.
The high-profile talks follow a day of diplomacy in Geneva as US Secretary of State John Kerry added his weight to efforts to resuscitate the stuttering truce.
Kerry said the situation in the war-torn country was "in many ways out of control and deeply disturbing" as the two-month-old ceasefire brokered by Washington and Moscow hangs by a thread.
Skirmishes continued in and around Aleppo, where more than 250 people have died during a week of violence.
Rebel rocket fire on government-held districts of the city killed at least four civilians including a child early on Tuesday, the Syrian Observatory for Human Rights monitoring group said.
State news agency SANA put the death toll at six and said 37 people were wounded.
However, an AFP correspondent reported no regime air strikes as ceasefire efforts intensify.
Meanwhile, heavy air strikes throughout the night on the Islamic State group's de facto Syria capital Raqa killed at least 13 civilians and five Islamist militants, the Observatory said.
But the monitoring group had no immediate word on whether the strikes were carried out by the Damascus regime, its Moscow ally or the US-led coalition battling IS.
"Raqa has not been targeted by air raids of this intensity for several weeks," Observatory chief Rami Abdel Rahman said.
"These raids continued throughout the night and into the morning."
Washington accuses Russia of doing little to rein in the forces of Syrian President Bashar al-Assad's around Aleppo.
But Moscow retorted earlier this week that it would not ask the Syrian regime to halt air raids on Aleppo as it believes that they are helping to combat Islamist militant groups there.
Since then, Russia has said talks are under way to include Aleppo province in a so-called "regime of silence" -- a freeze in fighting.
To buttress the truce, Washington and Moscow have agreed to bolster the number of Geneva-based ceasefire monitors, Kerry told reporters, pledging to work "in the next hours" to rein in violence on the ground.
Kerry accused Assad's regime of deliberately targeting three clinics and an attack on a major hospital last week, which he described as "unconscionable".
"It has to stop."
Kerry said a bolstered group of ceasefire monitors will track violations "24 hours a day, seven days a week".
A senior US diplomat, speaking on condition of anonymity, said the US, Russia and the UN had moved forward on a new ceasefire mechanism for Aleppo, but that the deal was not complete.
Kerry stressed that the goal was to reinforce a broad truce capable of withstanding further tests.
"We're trying to press this as fast as possible but I don't want to make any promises that can't be kept," he told reporters after meeting de Mistura and Saudi Foreign Minister Adel al-Jubeir -- whose government has influence with key rebel groups.
Before leaving Geneva en route to Washington, Kerry called Lavrov to discuss the flagging truce. The pair "agreed on new measures to be taken by Moscow and Washington", a Russian foreign ministry statement said, without providing details.
Kerry said Washington would press moderate rebels to separate themselves from the Al-Nusra Front's militants in Aleppo -- in a nod to Moscow's demands.
Russia and Assad's regime have used the presence of Al-Nusra, which was not party to the February 27 ceasefire deal, as an excuse to press their offensive.
The Syrian Observatory for Human Rights says at least 253 civilians -- including 49 children -- have been killed on both sides of divided Aleppo since April 22.
UN Secretary-General Ban Ki-moon appealed for the ceasefire to be extended to include Aleppo as a matter of urgency. The city was initially left out of a new deal announced last week to "freeze" fighting along two major fronts.
That freeze was extended until 1:00 am Wednesday (2200 GMT Tuesday) in Eastern Ghouta, state media said, and until 1:00 am Tuesday in Latakia, a regime stronghold.
Syria's conflict erupted in 2011 after anti-government protests were put down with the situation escalating into a multi-faceted war that has killed more than 270,000 people.
Last month's peace talks in Geneva failed to make headway, although de Mistura has voiced hope they can resume next month.