UN chief Ban Ki-moon is “appalled by the chilling military escalation” in the embattled Syrian city of Aleppo, his spokesman says.
Stephane Dujarric said the secretary-general was alarmed by reports of air strikes involving incendiary weapons and bunker-busting bombs.
The Syrian government has stepped up strikes on rebel-held areas of the city since a ceasefire collapsed last week.
The UN Security Council is due to meet on Sunday morning in New York.
The meeting was requested by the US, the UK and France.
Meanwhile, UK Foreign Secretary Boris Johnson has suggested that a deadly attack on an aid convoy in Syria last week could have been deliberately carried out by Russian aircraft.
If so, he said it could amount to a war crime. Russia has denied carrying out the attack, blaming instead rebel artillery.
Mr Dujarric said in a statement: “Since the announcement two days ago by the Syrian army of an offensive to capture eastern Aleppo, there have been repeated reports of air strikes involving the use of incendiary weapons and advanced munitions such as bunker-buster bombs.
“The secretary-general considers this a dark day for the global commitment to protect civilians.”
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Aleppo: Key battleground in Syria’s war
The northern city of Aleppo has become a key battleground in Syria’s bloody five-year civil war.
Last Tuesday, Mr Ban launched a stinging attack on the Syrian government, saying it had killed the most civilians in the conflict.
Media captionA baby boy is rescued from the rubble in Aleppo after a second day of heavy bombing, as Quentin Sommerville reports
Meanwhile, Syrian Foreign Minister Walid al-Moalem has said that government forces are making great strides against “terrorism”.
He told the UN General Assembly in New York on Saturday that his belief in victory was now greater than ever.
Syria refers to all rebel groups trying to overthrow President Bashar al-Assad as terrorists.
Earlier, Russian Foreign Minister Sergei Lavrov said that any revival of the cessation of hostilities in Syria could not depend on unilateral concessions by his country alone.
He said there had to be a collective effort involving all sides.
Media captionIntensified attacks on Aleppo have left nearly two million people without water.
The UN says the attacks on Aleppo have left nearly two million people without water.
Unicef, the UN children’s agency, has warned that fierce air strikes on Friday stopped repairs to a damaged water pumping station supplying rebel-held eastern districts of the city.
In retaliation, Unicef says, a nearby station pumping water to the west of Aleppo has been switched off.
Unicef spokesman Kieran Dwyer said water was being used as a weapon of war by all sides.
He said residents now had to resort to contaminated water and were at risk from waterborne diseases.
UK-based group monitoring the Syrian Observatory for Human Rights said 25 people were killed in fresh bombardments on Saturday.
Activists say both Syrian and Russian warplanes are taking part in the latest offensive, though Russia has not confirmed its involvement.
Russia supports the Syrian government, while the US backs the opposition. The two powers accuse each other of failing to rein in their respective allies on the ground.
