Syria's regime and its ally Russia Tuesday launched a wide-ranging assault on rebels with renewed strikes on the besieged eastern neighbourhoods of Aleppo, sparking fury in Washington.
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 said.
The Britain-based Syrian Observatory for Human Rights said at least five civilians were killed in the bombardment, the first of its kind since October 18.
An AFP correspondent in east Aleppo said strikes hit the Sakhur, Fardos and Masakan Hanano neighbourhoods while ambulances sped through the streets to evacuate the wounded.
Warplanes dropped flares to counter heat-seeking missiles, he said.
State Department spokeswoman Elizabeth Trudeau said Washington had received reports that the latest Russian and Syrian regime strikes had hit hospitals and clinics.
"We strongly condemn the resumption of air strikes in Syria by the Russians as well as the Syrian regime," she told reporters.
"The most recent reported attacks are on five hospitals and one mobile clinic in Syria. We believe it's a violation of international law."
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.
- Warning text messages -
The respite came after international criticism of a ferocious assault launched by Syrian and Russian forces on September 22 in a bid to recapture eastern Aleppo.
Civilians were among hundreds of people killed in the bombing campaign, which also destroyed infrastructure.
Moscow has organised several brief truces to encourage residents and surrendering rebels to leave east Aleppo, but few have gone.
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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.
On Sunday, east Aleppo residents received text messages from the army warning rebels to leave within 24 hours.
"After the end of this period, the planned strategic offensive will begin," the message said.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that has killed more than 300,000 people across the country since it started in March 2011 with anti-government protests.
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."
"Russia and the regime are trying to foil the efforts of the United Nations and the international community to reduce violence and protect civilians and deliver aid to civilians in all the besieged areas."
- Aircraft carrier -
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.
"For the first time in our naval history, the aircraft carrier Admiral Kuznetsov started taking part in combat," Shoigu said.
"Today from this carrier, our Sukhoi-33 (bombers) began their operations."
Russia has been carrying out a bombing campaign in Syria for the past year and deployed a naval contingent to back up its operation.
The Admiral Kuznetsov, Russia's sole aircraft carrier, arrived in the eastern Mediterranean off the Syrian coast as part of a flotilla of ships sent to reinforce Russia's military in the area.
Shoigu said the operation aimed to inflict heavy damage on "the Islamic State group and the Al-Nusra Front's positions in Idlib and Homs provinces."
Al-Nusra Front is the old name of Al-Qaeda's former Syrian affiliate, now known as Fateh al-Sham Front.
An AFP correspondent in Idlib said strikes hit several parts of the rebel stronghold province, including Maaret al-Numan, Saraqib and Ariha.
Idlib is largely controlled by a powerful rebel alliance that includes Fateh al-Sham.
The assault is the first major military development since the presidential election of Donald Trump, who has suggested working more closely with Russia in Syria.