The United States declared Friday that the bombing of civilians in Aleppo could amount to a war crime, as the diplomatic battle over the Syrian war moved to the United Nations.
US Secretary of State John Kerry made clear his mounting anger at the Russian-backed regime onslaught, declaring: "These are acts that beg for an appropriate investigation -- war crimes."
France, meanwhile, was preparing to call a vote on Saturday on a United Nations Security Council resolution to demand an end to Russia and Syria's aerial bombardment of Aleppo -- a motion that Russia is all but certain to veto.
Russia countered with a draft resolution of its own, calling for a ceasefire and demanding access for humanitarian aid, but with no mention of restraining air power over the city.
President Bashar al-Assad's forces launched their assault on eastern Aleppo, home to around a quarter-of-a-million civilians, two weeks ago after a US-Russian brokered ceasefire fell apart.
The United States has since abandoned bilateral talks with Russia on restoring the truce, but a new effort to revive a multilateral peace-building effort has made no progress as fighting rages.
Meeting in Washington with France's Foreign Minister Jean-Marc Ayrault, Kerry said Assad and Russia's jets had repeatedly and deliberately launched deadly strikes on hospitals.
"And those that commit these will be and should be held accountable for their actions," Kerry told reporters.
"This is a targeted strategy to terrorize civilians and to kill anybody and everybody who is in the way of their military objectives."
The blunt warning came amid soaring tensions, as US officials accused Russia of trying to manipulate the US election campaign through computer hacking and vowed to respond.
Ayrault said he would head to the United Nations on Saturday to make the case for a resolution demanding a ceasefire and a ban on Russian and Syrian jets overflying the city.
France's top diplomat, who met his Russian counterpart Sergei Lavrov on Thursday, insisted that hope was not dead that the move could go somewhere, despite the clear threat of a Russian veto.
But Moscow's UN Ambassador Vitaly Churkin told reporters: "I cannot possibly see how we can let this resolution pass."
The Russian text, obtained by AFP, "urges immediate implementation of the cessation of hostilities, in particular in Aleppo" and demands deliveries of humanitarian aid.
US officials are working on possible sanctions against Russian and Syrian individuals and companies involved in the siege, and Germany said it would not rule out backing possible international moves.
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Once Syria's economic powerhouse, Aleppo has been ravaged by the war that has killed more than 300,000 since it began in March 2011.
At least 250,000 people remain in the rebel-held east of the city. They have been under near-continuous siege for months, and are now facing some of the most intense bombardment yet.
Raids earlier this week destroyed the largest hospital in the rebel-controlled east.
- Indefinite deployment -
The talks in Washington came after UN special envoy Staffan de Mistura warned that Aleppo could be totally destroyed before the end of the year.
De Mistura also called on the regime and Russia to halt strikes if fighters from the former Al-Nusra Front, now known as Fateh al-Sham Front, left the city, even offering to escort them out himself.
Lavrov said he could support a UN plan for the jihadist faction to leave Aleppo if other opposition groups renounced their ties to the former Al-Qaeda subsidiary.
Russia's parliament on Friday ratified a deal with Syria on the "indefinite" deployment of its forces in the country.
In a move seen as firming up Russia's long-term presence, lawmakers approved an accord signed last year that could allow Moscow to establish a permanent presence at its Hmeimim Air Base.
- Aleppo life 'unbearable' -
Fierce fighting has gripped several neighborhoods on the frontline dividing rebel groups from regime forces in western areas.
Government troops captured a hilltop south of the city, but rebels retook nearby positions, according to British-based watchdog the Syrian Observatory for Human Rights.
Carlos Francisco, who heads the Syria branch of medical aid agency Doctors Without Borders, described the circumstances in Aleppo as "unbearable."
"The few remaining doctors with capability to save lives are also confronting death," he said.
Rebels have also intensified their rocket fire on government-held western Aleppo, where four people were killed in the Midan neighborhood, state television reported.