UNITED NATIONS -  Russia on Friday threatened to use its veto to block a French-drafted UN resolution calling for a ceasefire in Aleppo and the grounding of all warplanes over the Syrian city.

"I cannot possibly see how we can let this resolution pass," Russian Ambassador Vitaly Churkin told reporters when asked whether he would resort to the veto.

The United States on Friday demanded a war crimes investigation into the ferocious bombing campaign of Aleppo, accusing Syria and its Russian ally of "terrorising" civilians in the devastated city.

US Secretary of State John Kerry spoke ahead of discussions on a draft UN Security Council resolution that would call for an end to the Russian-backed onslaught on Aleppo.

The two-week assault by President Bashar al-Assad's forces has sparked a global outcry after air strikes on hospitals and other civilian infrastructure.

"These are acts that beg for an appropriate investigation - war crimes. And those that commit these will be and should be held accountable for their actions," Kerry told reporters in Washington.

He said Moscow and Damascus "owe the world more than an explanation about why they keep hitting hospitals and medical facilities, children and women. "This is a targeted strategy to terrorise civilians and to kill anybody and everybody who is in the way of their military objectives."

Germany also said it would not rule out backing possible sanctions against Russia over its bombardment of Aleppo.  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 east Aleppo, under near-continuous siege for months and now facing some of the most intense bombardment yet.

Raids earlier this week destroyed the largest hospital in the rebel-controlled east, and Kerry said on Friday another strike on a medical facility overnight killed 20 people. The Syrian Observatory for Human Rights, which monitors the conflict, could not confirm such an attack.

In New York, the UN Security Council began an emergency meeting on the war at Russia's request.

"The top priority is to stop the bloodbath in Aleppo," French Ambassador Francois Delattre told reporters as he headed into closed-door talks.

Security Council members have discussed for a week a French-drafted UN resolution calling for a ceasefire.

French Foreign Minister Jean-Marc Ayrault in Washington warned that an expected Saturday vote on the proposed Aleppo truce was "a moment of truth for all members of the Security Council."

Friday's talks come after UN special envoy Staffan de Mistura warned 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.

Russian Foreign Minister Sergei Lavrov said Friday he could support a UN plan for the jihadist faction to leave Aleppo if other rebel groups renounced their ties to the group.

"If Nusra leaves with its arms in the direction of Idlib... then for the sake of saving Aleppo we are ready to support such an approach and would be ready to call on the Syrian government to agree to this," Lavrov told Russian television.

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 Syrian presence, the vast majority of lawmakers approved an accord signed last year that could allow Moscow to establish a permanent presence at its Hmeimim airbase in northwest Syria.

Syria's army said earlier this week it would rein in its bombardment of east Aleppo, but has pressed on with its ground offensive.

Fierce fighting on Friday gripped several neighbourhoods on the front line dividing rebel groups from regime forces in western areas.

Government troops captured a hilltop south of the city, but rebels retook nearby positions from regime forces, according to the Observatory.

Carlos Francisco, the head of Doctors Without Borders (MSF)'s Syria branch, described circumstances in Aleppo as "unbearable".

"The few remaining doctors with capability to save lives are also confronting death," he said, adding that the manager of one MSF-supported facility was killed alongside his family by a barrel bomb several days earlier.

Rebels have also intensified their rocket fire on west Aleppo, where four people were killed in the Midan neighbourhood on Friday, state television reported.

In an interview with Danish broadcaster TV2 aired on Thursday, Assad said Aleppo's "best option" would be a local truce with the government.

Opposition forces say those deals have been forced upon them by a government strategy of "surrender or starve."

Assad said that without an agreement, he would "continue the fight with the rebels till they leave Aleppo... There's no other option."

Syria's White Helmets on Friday congratulated Colombia's president on winning the Nobel Peace Prize, which the rescue force had been widely tipped to receive themselves.

"For us, saving a life remains the most important prize that we could receive," the group's head Raed Saleh told AFP.