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Dozens of civilians were killed as air strikes flattened residential buildings in rebel-held east Aleppo Monday, despite Western warnings of sanctions against Syria and Russia over attacks on the city.

Once Syria's thriving commercial hub, Aleppo has been ravaged by bombing raids and intensifying clashes as President Bashar al-Assad's forces fight to capture opposition-held parts of the city.

Russian air support of the onslaught -- which has destroyed hospitals and other civilian infrastructure -- has spurred accusations of potential war crimes and threats of punitive sanctions.

World powers are expected to follow up on Monday on last-ditch diplomatic efforts at the weekend aimed at ending the conflict, which has killed more than 300,000 people since it erupted in March 2011.

The latest air strikes on Aleppo killed 13 civilians in the rebel-held district of Marjeh, taking to 46 the number left dead in a 24-hour period, said the Syrian Observatory for Human Rights.

Among them were nine children, including a 17-year-old and her newborn baby.

An AFP correspondent said White Helmets rescuers were working to pull more than a dozen people out of the rubble in the hardest-hit district of Qaterji, where the Observatory said Russian raids killed 17 civilians.

"Russian aircraft carried out a massacre in the Qaterji neighborhood by bombing a five- or six-floor building," said ambulance driver Abu Mohamed.

'It's a disaster'

"There are still families under the ruins. We pulled out seven or eight bodies and dozens of wounded. The hospitals are full of casualties. It's a disaster."

One of those rescued was Maarouf, a 12-year-old who had been trapped on the upper floor of a residential building that had collapsed on his legs.

A White Helmet volunteer in a crane could be seen approaching the boy, who was clad in a dust-covered shirt and wriggling in vain to get himself free.

Maarouf was extracted and his injured right leg was treated at a field hospital.

But White Helmets rescuers later emerged with a dead baby cradled in a blanket, her face caked in dust.

Both Russian and Syrian warplanes are carrying out air strikes over Aleppo in support of a major offensive by regime forces to capture rebel-held parts of the northern city.

More than 430 people have been killed in air raids and bombardment on the eastern half since the assault was announced on September 22, the Observatory said.

Another 82 people have died in rebel fire on government-held neighborhoods in the west, said Observatory director Rami Abdel Rahman.

In response to the onslaught, the United States and Britain warned at the weekend that they were considering new economic sanctions against Syria and Russia.

"There are a lot of measures that we're proposing including extra measures on the regime and their supporters," said British Foreign Secretary Boris Johnson.

"These things will eventually come to bite the perpetrators of these crimes, and they should think about it now."

'Extra measures' on Moscow?

But the European Union's foreign affairs head Federica Mogherini said the bloc of countries was not yet considering sanctions against Russia.

"This has not been proposed by any member state," Mogherini said as she arrived for a meeting of EU foreign ministers dominated by the Syrian crisis.

"But we have sanctions on the Syrian regime... and there are discussions on that, for sure, that could be possible."

US Secretary of State John Kerry met with his counterparts from Moscow, Tehran, and Syria's neighbours in Switzerland on Saturday, then travelled to London for meetings with Johnson and other European officials.

Kerry said the Lausanne meeting produced some new ideas on reviving a Syrian truce but said high-level contacts to flesh them out would continue Monday.

Syria's war has devolved from a widespread protest movement against Assad's rule to a multi-front war that has carved the country into zones of control held by rebels, jihadists, Kurds, and regime forces.

Turkish-backed rebel fighters dealt a major symbolic blow to the Islamic State jihadist group on Sunday by overrunning the town of Dabiq in Syria's north.

Dabiq holds crucial ideological importance for IS and its followers because of a Sunni prophecy that states it will be the site of an end-of-times battle between Christian forces and Muslims.

In June 2014, IS declared a self-styled Islamic caliphate across swathes of territory in Syria and Iraq.

Iraqi Prime Minister Haider al-Abadi announced earlier Monday that the long-awaited operation to recapture Mosul -- IS's last major stronghold in Iraq -- was under way.