Russia and the United States clashed at the United Nations over the carnage in Syria, as air strikes pounded Aleppo following the collapse of a ceasefire.
An angry US Secretary of State John Kerry demanded at a UN Security Council meeting that Russia force Syria to ground its air force, which Washington blames for an attack on an aid convoy.
Kerry and his Russian counterpart Sergei Lavrov are to sit down with key players in the conflict on Thursday to try to revive the ceasefire and chart a new course towards ending the five-year war.
"We are at a make or break moment," UN Secretary General Ban Ki-moon told the council, urging world powers to use their influence to help restart political talks so Syrians can "negotiate a way out of the hell in which they are trapped."
Russia and the United States negotiated the latest ceasefire plan, but Syria ended the truce on Monday following an apparently accidental US-led coalition strike on Syrian soldiers.
Shortly after the truce ended, the UN aid convoy was hit, killing 20 humanitarian workers and destroying 18 trucks carrying food for desperate civilians in Aleppo province.
On Wednesday, heavy bombardment pummeled Aleppo city and the wider province, key battlegrounds in Syria's conflict, and a raid hit a medical team late Tuesday.
Addressing the council, Kerry said the bombing of the aid trucks raised "profound doubt" about whether Russia and its Syrian ally were committed to upholding a ceasefire.
"We must move forward to try to immediately ground all aircraft flying in those key areas in order to de-escalate the situation and to give a chance for humanitarian assistance to flow unimpeded," he said.
Moscow denies that Russian or Syrian planes carried out Monday's strike on the aid convoy.
A Russian military spokesman said a coalition drone was in the area when the aid trucks were destroyed, a claim the Pentagon denied.
After halting aid operations in response to the convoy attack, the United Nations said it was ready to resume humanitarian deliveries.
- Diplomacy still in play -
Despite the US-Russian acrimony, diplomatic efforts were set to continue in New York with a new meeting of the International Syria Support Group (ISSG) to be held on Thursday.
The ISSG, which includes all key players in the Syria conflict, met for an hour on Tuesday, but made little headway in agreeing on the next steps to end the war that has killed 300,000 people.
Sounding a cautious note, Kerry told reporters late Wednesday that "it's going to be difficult. We'll see what people are willing to do."
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Moscow meanwhile said it is dispatching its flagship aircraft carrier, the Admiral Kuznetsov, to bolster its forces in the eastern Mediterranean off Syria.
In his address to the Security Council, Lavrov declared that there would be "no more unilateral pauses" by Syrian government forces, arguing that opposition fighters on the ground had previously used those ceasefires to regroup.
The foreign minister insisted that all sides must rein in rebel groups on the ground to ensure they comply with the ceasefire and said a list of terror groups not covered by the truce should be reviewed.
Only the Islamic State group and the Al-Qaeda-linked Nusra Front are not covered by the ceasefire, but Russia has long argued that other groups are fighting alongside those jihadists.
"If we can agree on this kind of comprehensive approach, an integrated, multifaceted approach, the chances of a cessation of hostilities surviving and being successful will be better," Lavrov said.
After hearing Lavrov, Kerry said he felt like the Russians were "sort of in a parallel universe" while the Russian foreign minister said it was time to "refrain from emotional reactions."
Syria's Ambassador Bashar Jaafari railed against a "filthy propaganda war" waged by the United States and its allies, insisting his government is fighting "tens of thousands of terrorists" backed by foreign countries.
- Aleppo under fire -
Dozens of raids hit the city's east as regime troops advanced on rebels in Aleppo's southwestern outskirts, according to the Britain-based Syrian Observatory for Human Rights.
At least 12 civilians, including two children, were killed, the highest single-day toll since the truce collapsed this week, the group said.
In northwest Idlib province, 18 Syrians were killed during bombings on the town of Khan Sheikhun, according to the organization.
In the village of Khan Tuman, south of Aleppo city, two nurses and two drivers were killed in an attack on two ambulances, the Union of Syrian Medical Relief Organizations said.
Syrian state media reported that the city's government-held west had come under rebel shelling, which killed two people.
The United States holds Russia responsible for the attack on the aid convoy, with a US official saying two Russian SU-24 ground attack jets were operating in the area where it was struck.
The Russian foreign ministry said the "unsubstantiated, hasty accusations" seemed designed to "distract attention from the strange 'error' of coalition pilots."
Dozens of Syrian troops were killed in Saturday's strike by the US-led coalition fighting Islamic State, which Washington said was a mistake.
Russia said it endangered the deal reached with Washington that provides for a ceasefire, humanitarian aid deliveries and cooperation between Moscow and Washington in battling IS and other extremist groups.