The Syrian military has announced a new offensive in rebel-held eastern Aleppo, where a quarter of a million people are living under siege.
Jets pounded rebel positions in the city on Wednesday night as a week-old truce collapsed, reportedly killing at least 13 people.
It is unclear whether the new offensive will involve ground troops.
Talks between the US and Russia on reviving the collapsed ceasefire have broken up without progress in New York.
Russia and the US support Syria’s government and opposition respectively.
The US Secretary of State, John Kerry, said Washington could not be the only one trying to hold open the door to peace. The US wants Russia to press the Syrian government to ground its warplanes.
Mr Kerry added that he would hold further consultations with the Russian team on Friday.
Russian Foreign Minister Sergei Lavrov said that would amount to a “unilateral pause”. Some had attempted to present the ceasefire as if only the Syrian government should be taking steps, he said, but the opposition also needed to stop fighting.
The UN envoy for Syria, Staffan de Mistura, described the meeting as “long, painful and disappointing”.
This feels like a very serious breakdown, with little obvious room left for dialogue until the major powers find new space for compromise, the BBC’s diplomatic correspondent, James Robbins, reports from New York.
The week in new York is heading towards disastrous diplomatic failure, he adds.
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‘New offensive’
Syrian state media quoted army officials as urging civilians in eastern parts of the city to avoid areas where “terrorists” were operating.
They said there were exit points for those who wanted to flee, including rebels.
Image copyright AFP
Image caption Rebel-held parts of Aleppo have been pounded by Syrian and Russian jets
Women and children are believed to be among those killed in the latest bombing. Some reports put the death toll as high as 45.
An AFP news agency journalist reports that his entire street in the southern Bustan al-Qasr district was left burning after warplanes dropped incendiary bombs.
The Syrian Observatory for Human Rights, a UK-based group of opposition activists, said 14 air strikes had hit Bustan al-Qasr and the neighbouring Kallasa district, starting “massive fires”, as rebels and troops clashed.
Rami Abdul Rahman, the director of the Observatory, described them as “the most intense strikes in months” on the two areas, with three women and three children among the dead.
Media captionUS Secretary of State John Kerry speaking to the UN about the fragile Syria ceasefire
The Aleppo Media Centre said the fires had been caused by “incendiary phosphorus bombs”. Video footage posted by it and another pro-opposition activist group, Thiqa, showed intense blazes lighting up the night sky.
Aleppo, once Syria’s commercial and industrial hub, has been divided roughly in two since 2012, with the government controlling the west and rebels the east
