Syrian government and Russian warplanes pounded rebel-held parts of northern Syria Wednesday, including battered second city Aleppo, where food aid rations were near-depleted after months of regime siege.
The renewed bombardment killed at least 35 people in Aleppo city in 24 hours, and sparked anger from Washington and the United Nations.
It came as President Bashar al-Assad said in an interview that US president-elect Donald Trump could be a "natural ally" if he fights "terrorists".
Damascus considers all those who oppose Assad's government to be "terrorists" like the Islamic State jihadist group, which Trump has said should be the focus of US involvement in Syria.
The regime and its ally Russia launched a wide-ranging assault on rebels on Tuesday, shattering a month of relative calm in the rebel-held east of devastated Aleppo.
But Moscow denied carrying out air strikes on east Aleppo.
"Planes of the Russian air force have not been carrying out strikes on Aleppo for the past 29 days," said Igor Konashenkov, a spokesman for the defence ministry.
The Syrian Observatory for Human Rights, a Britain-based monitor, said six children were among the 35 killed in government strikes and artillery fire on besieged opposition-held districts since Tuesday.
The Independent Doctors Association, a medical group, said barrel bomb attacks had damaged two facilities it supports in eastern Aleppo -- the children's hospital and the only blood bank in the area.
Medical facilities have regularly been hit, and sometimes completely destroyed, in the government's fight against rebels, though Damascus and Moscow deny they target hospitals.
- Food aid runs out in Aleppo -
The Observatory also reported 21 people were killed in Batabo, a village on the border between Aleppo and Idlib province, where Russia and Syria were carrying out strikes.
The monitor said it was not immediately clear if the strikes on Batabo were carried out by Russian or Syrian planes.
The bombardment ended a period of relative respite, particularly in eastern Aleppo, where Moscow halted air strikes on October 18 ahead of a series of brief ceasefires.
The ceasefires were intended to encourage residents and surrendering rebels to leave the east, but few did so, fearing moving into government-held territory.
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Food aid stockpiled in the east is all-but-exhausted, with international organisations and their local partners saying they have distributed final rations in recent days.
No aid has entered the eastern neighbourhoods since government troops surrounded them in mid-July.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that has killed more than 300,000 people nationwide since it started in March 2011 with anti-government protests.
Russia intervened in September 2015 in a bid to bolster the government, and Tuesday said its forces were launching a "major operation" in Idlib and central Homs province, targeting IS and former Al-Qaeda affiliate Fateh al-Sham Front with air strikes from a carrier in the eastern Mediterranean.
But the bombardment has been criticised by both the UN and Washington, with the General Assembly's human rights committee voting overwhelmingly Tuesday to condemn escalating attacks on civilians.
- Assad open to Trump alliance -
Washington said it had received reports that the latest bombing raids had damaged civilian infrastructure in rebel areas.
"We strongly condemn the resumption of air strikes in Syria by the Russians as well as the Syrian regime," State Department spokeswoman Elizabeth Trudeau said.
"The most recent reported attacks are on five hospitals and one mobile clinic in Syria. We believe it's a violation of international law."
Washington was an early backer of the uprising against Assad, and has supported the rebels fighting his government.
But that could change under the next administration, with Assad telling Portugal's RTP state television Tuesday he welcomed Trump's campaign comments suggesting Washington's involvement in Syria should be focused exclusively on fighting jihadists.
"We cannot tell anything about what he's going to do, but if... he is going to fight the terrorists, of course we are going to be ally, natural ally in that regard with the Russian, with the Iranian, with many other countries," Assad said.
Washington already leads an international coalition carrying out strikes against IS in Syria and Iraq, but it does not coordinate with Damascus and Assad's government has condemned it as ineffective.
The coalition is supporting an operation by the Kurdish-Arab alliance known as the Syrian Democratic Forces to capture the IS bastion of Raqa.
"We are advancing even though IS is mining the villages as they flee," SDF commander Rodi Derik said Wednesday in the village of Tuwaylaa, recently captured from IS.