UN Secretary-General Ban Ki-moon said he was appalled by the "chilling" upsurge in fighting in Aleppo and warned that the use of advanced weaponry in Syria's battleground city could amount to war crimes.
Mr Ban cited reports of air strikes involving the use of incendiary weapons, bunker-buster bombs and other powerful munitions in the offensive launched by the Syrian army two days ago to recapture the city.
He warned that "the apparent systematic use of these types of indiscriminate weapons in densely populated areas may amount to war crimes," a statement from his spokesman said.
The UN chief said he was "appalled by the chilling military escalation in the city of Aleppo, which is facing the most sustained and intense bombardment since the start of the Syrian conflict".
The Syrian army and militia allies seized ground north of Aleppo today, tightening a siege of the city's rebel-held east while warplanes bombed it relentlessly in a Russian-backed offensive.
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The capture of Handarat, a Palestinian refugee camp a few kilometres north of Aleppo, marked the first major ground advance of the offensive, which the government announced on Thursday.
The camp, on elevated ground overlooking one of the main roads into Aleppo, had been in rebel hands for years.
"Handarat has fallen," an official with one of the main Aleppo rebel groups said. A Syrian army statement confirming the advance said "large numbers of terrorists" had been killed.
The assault on Aleppo, where more than 250,000 civilians are trapped in a besieged opposition sector, could be the biggest battle yet in a civil war that has killed hundreds of thousands of people and driven 11 million from their homes.
Two weeks after Moscow and Washington announced a ceasefire, President Bashar al-Assad and his Russian and Iranian allies appear to have launched a campaign for a decisive battlefield victory that has buried any hope for diplomacy.
Dozens of people have been reported killed in eastern Aleppo since the army announced the new offensive.
Residents say air strikes on eastern Aleppo have been more intense than ever, using more powerful bombs.
Rebel officials said heavy air strikes today hit at least four areas of the opposition-held east, and they believe the strikes are mostly being carried out by Russian warplanes.
The Syrian Observatory for Human Rights said at least 45 people, among them 10 children, were killed in eastern Aleppo today. Rescue workers said yesterday's death toll was over 100.
The Syrian army said it is only targeting militants in the campaign announced on Thursday evening.
The war has ground on for nearly six years, with all diplomatic efforts collapsing in failure.
Half of Syria's population has been made homeless, the war has drawn in world powers and regional states, and the so-called Islamic State - the enemy of all other sides - seized swathes of Syria and neighbouring Iraq.
For most of that time, world powers seemed to accept that neither Mr Assad nor his opponents was likely to be capable of decisive victory on the battlefield.
But Russia's renewed involvement this week could reflect a change in that calculus and a view that victory is in reach, at least in the western cities where the overwhelming majority of Syrians live.
Mr Assad's fortunes improved a year ago when Russia joined the war on his side. Since then, Washington has worked hard to negotiate peace with Moscow, producing two ceasefires.
But both proved short-lived, with Mr Assad, possibly scenting chances for more battlefield success, showing no sign of compromise.