The United States strongly condemned Russia's renewed bombing campaign, warning that strikes on civilian targets break international law.
Russian Defence Minister Sergei Shoigu spoke of a "major operation" which saw the first missions carried out by warplanes taking off from the Admiral Kuznetsov aircraft carrier that arrived off Syria last week.
In Aleppo, Syrian government aircraft pounded the eastern neighbourhoods with air strikes and barrel bomb attacks, a monitor and AFP correspondent yesterday said.
State Department spokeswoman Elizabeth Trudeau said Washington had received reports that the latest Russian and Syrian regime strikes had hit hospitals and clinics.
More From This Section
UN member-states also overwhelmingly condemned escalating attacks against civilians in Aleppo and called for a ceasefire to pave the way to a settlement for Syria's five-year war.
The assault ended a period of relative respite for more than 250,000 people living in besieged eastern Aleppo.
On October 18, Moscow said it was halting its air strikes ahead of a short-lived truce and Syrian raids also subsided, with bombardment mostly confined to areas where clashes were taking place on the edges of the battered city.
Civilians were among hundreds of people killed in the bombing campaign, which also destroyed infrastructure.
No aid has entered eastern Aleppo since it was first surrounded by government troops in mid-July, and the UN said Thursday it was distributing its last remaining food rations in the rebel districts.
The UN's food agency, FAO, said a farming crisis across Syria had reduced food production to a record low and raised fears that people would be forced to flee famine.
"After the end of this period, the planned strategic offensive will begin," the message said.
Yasser Yusef, of the Nureddine al-Zinki rebel group, said the bombardment would lead to "massacres and crimes against humanity in northern Syria and particularly in Aleppo city and Idlib."
As the strikes hit Aleppo, Moscow announced its Admiral Kuznetsov aircraft carrier was now being used in its campaign to bolster President Bashar al-Assad.
Russia has been carrying out a bombing campaign in Syria for the past year and deployed a naval contingent to back up its operation.