There has been progress on a plan to restore Syria's fraying ceasefire, but last details still need to be worked out among global powers, US Secretary of State John Kerry said Monday in Geneva.
"We are hopeful, but we are not yet there yet," he told reporters after meeting Saudi Foreign Minister Adel al-Jubeir and UN Syria peace mediator Staffan de Mistura.
Kerry held the Syrian Army responsible for attacks on hospitals and civilians amid the escalating violence in Aleppo city but made clear that "both sides - the opposition and the regime - have contributed to this chaos."
Russia and Iran, which support Syria's armed forces, must now make sure that the government of Syrian President Bashar al-Assad honours the truce, while the United States and other countries that back moderate rebels must do the same on the other side, Kerry said.
"We're trying to press this as fast as is possible," he said, expressing hope that the extent of progress would become clearer on Tuesday.
The US chief diplomat later spoke with Russian Foreign Minister Sergei Lavrov by phone Monday, Russian news agency TASS reported.
"Both ministers continued discussing the prospects for settlement and urged the parties to the conflict to observe ceasefire," a Russian Foreign Ministry statement said, according to the report.
De Mistura said he would hold talks Tuesday in Russia.
UN Secretary General Ban Ki-moon was "profoundly concerned" about the renewed fighting, calling on all sides to "recommit immediately" to the ceasefire and protect civilians, Stephane Dujarric, Ban's spokesman, said in New York.
"The collapse of the cessation of hostilities will only bring more violence, death and destruction, while further weakening efforts to find a negotiated solution to this brutal war," Dujarric said.
The recent surge in Syria's violence has scuttled a ceasefire that was brokered by Washington and Moscow and that went into effect in most parts of the country in late February.
De Mistura ended his latest round of peace consultations last week in Geneva, after opposition delegates halted their participation to protest the increased fighting and the regime's blocking of humanitarian aid.
Kerry said several plans for reviving the truce are on the table, but some details have yet to be agreed and the proposals still "need to be signed off by some people."
Saudi Arabia's al-Jubeir declared that the world would not stand by as Syrian President Bashar al-Assad and his foreign allies massacre civilians in Aleppo.
"The world is not going to allow them to get away with this," al-Jubeir said. "Bashar al-Assad's days are numbered."
Al-Jubeir warned that the Syrian president would have to resign or face a violent ouster.
Saudi Arabia has been one of the most important backers of the Syrian opposition, including armed groups, and hosts the main opposition bloc that has taken part in the Geneva peace talks.
The Syria government said Sunday it was extending a ceasefire in and around Damascus, while activists reported heavy fighting betweeb the regime and rebels in Aleppo.
The temporary ceasefire excludes Aleppo, where rebels fired shells Sunday on regime-held areas and unidentified helicopters dropped barrel bombs on rebel-controlled areas.