The top US diplomat gave some of his most downbeat comments yet after meeting the UN peace envoy on Syria, saying the conflict was "in many ways out of control and deeply disturbing to everybody in the world, I hope."
"The attack on this hospital is unconscionable," he said, accusing President Bashar al-Assad's regime of deliberately targeting three clinics and a major hospital last week. "And it has to stop."
Washington and Moscow are the joint sponsors of the Syrian peace process, and de Mistura has made it clear that he sees little hope of progress without their agreement.
Kerry said he would call Russian Foreign Minister Sergei Lavrov later Monday to press for the ceasefire to be restored.
De Mistura was due to fly to Moscow for talks with Lavrov on Tuesday.
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 Syrian city.
"What is happening in Aleppo is an outrage. It's a violation of all humanitarian laws. It's a crime," said Saudi Foreign Minister Adel al-Jubeir as he met Kerry.
"It's a violation of all the understandings that were reached," he added, accusing Assad and the Russians of violating international agreements to back peace.
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.
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