Warplanes bombed Aleppo on Friday with what residents described as unprecedented ferocity after the Russian-backed Syrian army announced an offensive to fully capture Syria's biggest city, killing off any hope of reviving a ceasefire.
The apparent collapse of U.S.-backed peacemaking may mark a turning point in the 5-1/2-year-old war, with the government and its Russian and Iranian allies now seemingly determined to crush the rebellion in its biggest urban stronghold.
The Syrian army announced overnight that it was launching an operation to recapture the rebel-held sector of the city. On Friday a Syrian military source denied the army was targeting civilians, saying it was accurately targeting "terrorist positions" in Aleppo.
The assault left no doubt that the government of President Bashar al-Assad and its Russian allies had spurned a plea from U.S. Secretary of State John Kerry to halt flights to resurrect the ceasefire, which collapsed on Monday after a week.
At the United Nations, diplomatic efforts to revive the truce made no tangible progress.
"My fear is that the bombings of the last few hours in Aleppo show that the regime is actually playing the card of partitioning Syria and its backers are letting it happen," French Foreign Minister Jean-Marc Ayraud told reporters.
Recovering full control of the rebels' last significant urban area would be the most important victory of the war so far for Assad, strengthening his control over Syria's most populous and strategically important regions.
In its late-night announcement on Thursday, the Syrian military announced "the start of its operations in the eastern districts of Aleppo". It warned residents to stay away from "the headquarters and positions of the armed terrorist gangs".
An army source said on Friday that the offensive would be "comprehensive", with a ground assault following air and artillery bombardment. "With respect to the air or artillery strikes, they may continue for some time," he said.
Several residents said explosions had struck with far greater force than anything that had hit the city in the past, making bigger craters and bringing entire buildings down.
The collapse of the ceasefire - the same fate as that of all previous efforts to halt a war that has killed hundreds of thousands of Syrians - appeared to have doomed the peace bid, probably the last chance for a settlement before U.S. President Barack Obama leaves office in January.
The Syrian government, strengthened by Russian air power and Iranian-backed Shi'ite militias, has been tightening its grip on rebel-held districts of Aleppo this year, and this summer achieved a long-held goal of fully encircling the area.
Syrian state television reported late on Friday that the army had managed to advance and secure control over a road near Ramousah in southwest Aleppo, but this was denied by rebels. They said several attempts by the army to advance had failed.
On Thursday in New York, the United States and Russia failed to agree on how to revive the ceasefire during what U.N. Syria mediator Staffan de Mistura called a "long, painful, difficult and disappointing" meeting.