US Secretary of State John Kerry launched a desperate push Monday to salvage a ceasefire in Syria, as the country's second city of Aleppo reeled from a week of fighting that killed hundreds of civilians.
With the two-month old truce brokered by the United States and Russia under severe threat, Kerry said Washington and Moscow had made progress in trying to contain the bloodshed, but warned it was premature to promise success.
The top US diplomat gave some of his most downbeat comments yet after meeting the United Nations Syria envoy Staffan de Mistura in Geneva, saying the conflict was "in many ways out of control and deeply disturbing to everybody in the world, I hope."
The US and Russia 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.
In and around Aleppo, a week of fighting has killed more than 250 people.
Kerry accused President Bashar al-Assad's regime of deliberately targeting three clinics and a major hospital last week.
"The attack on this hospital is unconscionable," he said. "And it has to stop."
There was a relative lull in the unrest later Monday, allowing some residents to venture out into the streets, AFP's correspondents in Aleppo said, with some even opening up shops.
Greengrocer Abu Nazem said he had not worked all of last week out of fear of air raids.
"I decided to come back to work today but despite the calm I am still afraid. Sometimes I hear motorbikes going past and I dive for cover because I think it's a military plane," he told AFP.
- Diplomatic push -
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 have moved forward on a new ceasefire mechanism for Aleppo, but a 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 Russian Foreign Minister Sergei Lavrov to discuss safeguarding the flagging truce.
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The pair "agreed on new measures to be taken by Moscow and Washington", a Russian foreign ministry statement said, without providing details.
De Mistura was due to fly to Moscow for talks with Lavrov on Tuesday.
- Aleppo violence -
While agreeing in theory to support a ceasefire, Russia has done little to rein in Assad's forces around Aleppo, which were in action again early Monday.
Several neighbourhoods, including the heavily-populated Bustan al-Qasr district, were hit, according to AFP's correspondent in the northern city.
Kerry said Washington would press moderate rebels to separate themselves from the Al-Nusra Front's jihadists in Aleppo.
Russia and Assad's regime have used the presence of Al-Nusra, which was not party to a February 27 ceasefire deal, as an excuse to press their offensive.
The Syrian Observatory for Human Rights monitor 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 deal to "reinforce" the February truce agreement.
The freeze in fighting, announced on Friday, applied to battlefronts in the coastal province of Latakia and Eastern Ghouta, near Damascus.
State television reported a Syrian army announcement on Monday that the freeze has been extended for another 48 hours in Eastern Ghouta, until 1:00 am Wednesday (2200 GMT Tuesday).
The same "freeze" is set to hold until 1:00 am Tuesday in Latakia, a regime stronghold.
Syria's conflict erupted in 2011 after the brutal repression of anti-government protests and has since escalated into a complex, multi-faceted war, which has killed more than 270,000 people.
The renewed bloodshed has dampened hopes that the ceasefire could finally lay the groundwork for an end to the conflict.
Last month's peace talks in Geneva failed to make headway, although de Mistura has voiced hope they can resume next month.