Syrian or Russian warplanes bombed aid trucks near Aleppo after a fragile week-long ceasefire ended as it appeared the bloody five-year war was fully back on late on Monday.
The UK-based Syrian Observatory for Human Rights said at least 32 people were killed in dozens of air strikes launched in and around Aleppo after the truce officially came to an end at 1600 GMT.
The war monitor said the aid trucks made a routine delivery to an area west of Aleppo city and were hit near the town of Urm al-Kubra, killing 12 people.
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An official with the Syrian Red Crescent confirmed aid vehicles operated by the group had been targeted by air strikes as warplanes resumed bombings in Aleppo province.
Staffan de Mistura, United Nations Special Envoy for Syria, denounced the air raid. "Our outrage at this attack is enormous... The convoy was the outcome of a long process of permission and preparations to assist isolated civilians," he said.
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Syria's military on Monday declared that the seven-day, US-Russian brokered ceasefire was over as the government and opposition traded accusations over mounting violations.
An AFP news agency correspondent in Aleppo reported the northern city was being pummeled.
Sirens wailed as ambulances zipped through the eastern rebel-held half of the divided city, the correspondent said, describing the bombardment as "non-stop".
The Russian military said rebels launched a major attack on a government position on Aleppo's southwestern outskirts, forcing Syrian troops to respond.
"The attack by the terrorists was proceeded by a massive artillery bombardment ... from tanks and rocket systems," it said.
The US said it's prepared to extend the fractured truce, and Russia - after blaming rebels for the violations - suggested it could still be salvaged.
In the wake of the Syrian military declaration, US Secretary of State John Kerry expressed annoyance at Damascus and Moscow's handling of the ceasefire.
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"It would be good if they didn't talk first to the press but if they talked to the people who are actually negotiating this," Kerry said. "As I said yesterday, [it's] time to end the grandstanding and time to do the real work of delivering on the humanitarian goods that are necessary for access."
But Kerry also acknowledged the first stage of the truce - which called for a week of calm and the delivery of humanitarian aid to several besieged communities - had never really come to fruition.
From the start, the truce had been beset by difficulties and mutual accusations of violations.
Aid deliveries to the besieged eastern districts of Aleppo have not reached their destination. The UN accused the government of obstructing the delivery, while Russian officials said rebels opened fire at the delivery roads.
At least 22 civilians were killed in government bombings over the last week, according to the Syrian Observatory, which uses a network of sources on the ground to compile its reports.
Earlier Monday, Russia's Lieutenant-General Sergei Rudskoi said in a briefing that Damascus had fulfilled its obligations.
"With the rebels failing to fulfill conditions the ceasefire agreement, we consider its unilateral observance by the Syrian government forces meaningless," Rudskoi said.
He said the rebels violated the truce 302 times since it took effect a week ago, killing 63 civilians and 153 Syrian soldiers. The opposition reported on Monday 254 violations by government forces and their allies.
The current tensions come on the heels of the weekend air strikes by the US-led coalition on Syrian army positions near Deir Az Zor.
Russia's military said at least 62 Syrian soldiers were killed and more than 100 wounded.
The US military said it would not intentionally hit Syrian troops, and it came as it was conducting a raid on ISIL positions.
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