The United States has accused Russia of "barbarism" in Syria on as warplanes supporting Syrian government forces pounded Aleppo and Russia said ending the civil war was almost "impossible".
A diplomatic solution to the fighting looked unlikely as US and Russian diplomats disagreed at a UN Security Council meeting called to discuss the violence, which has escalated since a ceasefire collapsed last week.
Rebels, who are battling President Bashar al-Assad's forces for control of Aleppo, said any peace process would be futile unless the "scorched earth bombing" stopped immediately.
Capturing the rebel-held half of Syria's largest city, where more than 250,000 civilians are trapped, would be the biggest victory of the civil war for Mr Assad's forces.
They have achieved their strongest position in years thanks to Russian and Iranian support and launched a fresh offensive for a decisive battlefield victory on Thursday.
Residents and rebels say thousands have been killed in the new strikes.
"What Russia is sponsoring and doing is not counterterrorism, it is barbarism," US Ambassador to the United Nations, Samantha Power, told the 15-member council.
"Instead of pursuing peace, Russia and Assad make war. Instead of helping get lifesaving aid to civilians, Russia and Assad are bombing the humanitarian convoys, hospitals, and first responders who are trying desperately to keep people alive."
Minister for Foreign Affairs Charlie Flanagan called on the Russian Federation to recognise that its military actions in Aleppo have had catastrophic consequences for vulnerable civilians and must cease.
He said a full ceasefire observed by all sides must be restored.
Mr Flanagan was speaking following his return from the United Nations General Assembly.
The French and British foreign ministers also took aim at Russia at the Assembly, saying it could be guilty of war crimes.
But Russia defended its position.
"In Syria hundreds of armed groups are being armed, the territory of the country is being bombed indiscriminately and bringing a peace is almost an impossible task now because of this," Russian UN Ambassador Vitaly Churkin told the council.
In the first major advance of the new offensive, Syrian forces seized control of the Handarat Palestinian refugee camp, north of Aleppo.
Rebels counter attacked and said yesterday they had retaken the camp before the bombing started.
"We retook the camp, but the regime burnt it with phosphorous bombs," said Abu al-Hassanien, a commander in a rebel operations room that includes the main brigades fighting to repel the army assault.
The army, which is also being helped by Iranian-backed militias, Lebanon's Shia Hezbollah militant group and a Palestinian militia, acknowledged rebels had retaken Handarat.
"The Syrian army is targeting the armed groups' positions in Handarat camp," a military source was quoted on state media as saying.
Planes continued to pound residential areas yesterday, flattening buildings, rebels and residents said.
"The Assad regime and with direct participation of its ally Russia and Iranian militias has escalated its criminal and vicious attack on our people in Aleppo employing a scorched earth policy to destroy the city and uproot its people," a statement signed by 30 mainstream rebel groups said yesterday.
The Syrian Observatory for Human Rights, a monitoring group, said at least 45 people, among them ten children, were killed in eastern Aleppo on Saturday.
The army says it is targeting only militants.
Hundreds of thousands of people have been killed in the civil war and 11 million driven from their homes.