UN Secretary-General Ban Ki-moon on Saturday 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.
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."
Intense air strikes toppled buildings and killed at least 45 civilians in Aleppo on Saturday while nearly two million people were left without water in the war-battered city.
Ban "considers this a dark day for the global commitment to protect civilians," said the statement.
He urged the international community to unite to send a clear message to the Syrian government that "it will not tolerate the use of indiscriminate and ever more deadly and powerful weapons against civilians."
Rebel-held districts in east Aleppo came under intense air and artillery fire for a fifth night as the army prepared a ground offensive to recapture the whole of the divided city.