UN Secretary-General Ban Ki-moon is "appalled by the chilling military escalation" in the northern Syrian city of Aleppo, his spokesman says.
"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," Stephane Dujarric said in a statement on September 24.
Ban "considers this a dark day for the global commitment to protect civilians," Dujarric added.
Damascus has stepped up strikes on rebel-held areas of the city since a cease-fire collapsed last week.
The UN Security Council is to meet in New York later on September 25 to discuss the escalation of violence, diplomats say.
The meeting was requested by Britain, France, and the United States.
UNICEF, the UN children's organization, says recent attacks have left nearly 2 million people without water.
A pumping station providing water for rebel-held eastern districts of Aleppo was destroyed by bombing, and the rebels responded by shutting down a station supplying the west of the city, according to UNICEF.
On September 24, the Britain-based Syrian Observatory for Human Rights said at least 45 people, among them 10 children, were killed in bombardments in eastern Aleppo.
"The raids are intense and continuous," according to the organization’s director, Rami Abdulrahman.
Activists said both Syrian and Russian warplanes were taking part in the offensive.
The Syrian Army said it was targeting rebel positions in the city and denied hitting civilians. Russia has not confirmed its involvement.
The Syrian military announced late on September 22 that it was starting a new operation against the rebel-held eastern part of the city, which is home to at least 250,000 people.
An army source was quoted as saying the offensive would be "comprehensive," with a ground assault following air and artillery bombardment.
On September 24 in New York, Syrian Foreign Minister Walid al-Moalem told the UN General Assembly that Damascus's belief in victory was now greater than ever, adding that government forces were "making great strides in its war against terrorism."
The government refers to all rebel groups fighting against President Bashar al-Assad's forces as terrorists.
Earlier, Russian Foreign Minister Sergei Lavrov said that a revival of a cease-fire in Syria depended on all sides involved and not only on "Russia's unilateral concessions."
"One can only speak about the cease-fire revival on the collective basis," he said in an interview for the TV news show Vesti.
He was speaking a day after meeting with U.S. Secretary of State John Kerry in New York for talks to reestablish a truce.
Kerry reported "little progress" after the meeting.
Moscow and Washington accuse each other of failing to rein in their respective allies in Syria -- where Russia supports Assad, while Washington backs opposition groups.
The five-year conflict has killed more than 300,000 people and driven millions of others from their homes.