The UN Security Council will hold an urgent meeting Sunday regarding increased bloodshed in Syria, Anadolu agency reported.
After a weeklong truce, the war came back raging in Aleppo where nearly 300,000 civilians are besieged and in desperate need of humanitarian assistance.
Secretary-General Ban Ki-moon is "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," a statement from the UN said.
The Syrian regime announced Thursday an offensive against the country's pre-war second city and reports have since surfaced of airstrikes involving the use of incendiary weapons and advanced munitions such as bunker buster bombs.
"The apparent systematic use of these types of indiscriminate weapons in densely populated areas may amount to war crimes," the statement added.
Describing the reported attacks as a "dark day" for the global commitment to protect civilians, Ban called on the international community to "unite to send a clear message" of no tolerance for the use of deadly weapons against civilians.
The regime's offensive came days after a truce ended catastrophically after a UN aid convoy was bombed before reaching Aleppo to deliver aid.
The regime vehemently protested an attack on its troops early last week when a US airstrike hit a regime unit and killed 62 soldiers.