Syrian government forces have begun an all-out ground assault on besieged rebel-held east Aleppo, as they attempt to retake the whole of the city.
Troops attacked from four fronts, in their first offensive inside the city since the opposition took hold in 2012.
They advanced towards the contested old quarter of Aleppo - home to the Umayyad Mosque, a UNESCO world heritage site and centerpiece of the city's walled Old City.
There was also fierce fighting to the south of Aleppo, where rebels are battling pro-government troops to reopen the only route out for the some 250,000 residents on the eastern side who remain trapped.
For the past five days, Syrian and Russian warplanes have been bombing east Aleppo with an unprecedented ferocity.
More than 350 people have been killed in the bombing campaign since Friday, half of whom were reported to be women or children.
Doctors, who have been unable to cope with the number of patients they are seeing, say hospitals are quickly running out of medicine and supplies. Save the Children said medics were having to operate on children without any anaesthetic as hospitals did not have enough left.
Jens Stoltenberg, the head of Nato, called the strikes "a violation of international law" that were "morally totally unacceptable."
"The regime is cowardly in battle," said Abdulkafi al-Hamdo, an English teacher living close to the front line in Aleppo. "So they burn everything from the skies first and then progress."
A ceasefire brokered by the US and Russia and billed as Syria's "last chance" collapsed last week amid recriminations and accusations of violations from both sides.
Convinced that any further talks with Washington were futile, Russia has decided to throw its military might behind the Syrian regime's drive to recapture Aleppo.
Aleppo, Syria's most populous city and its former industrial hub, has been the location of some of the fiercest violence of the five-year conflict. The city is so important to both sides that it is said whoever takes it wins the war.
A defeat for the rebels in Aleppo would likely deal a knockout blow to the opposition, which would be wiped out of all major cities. A win for the regime would embolden President Bashar al-Assad and put him in better standing for any future negotiations.
A Western diplomat said he feared that with neither side strong enough to hold Aleppo, there could be up to a year of street-to-street fighting. "The only way to take eastern Aleppo is by such a monstrous atrocity that it would resonate for generations," he told the Telegraph.
The Syrian army, which is thought to number approximately 15,000-20,000 around Aleppo, is bolstered by tens of thousands of troops from Russia, Iran and Iraq, as well by Shia militias.
There are estimated to be a similar number of opposition fighters, who have formed unlikely alliances in an attempt to hold back government forces.
Ammar al-Sakka, the spokesman of the moderate Fastaqim rebel group, told the Telegraph they would fight to the death for Aleppo, the "castle of resistance".
He said they needed more weapons, however the siege made it impossible.
"The regime plan for Aleppo looks to be: besiege, starve, bombard, terrify, and shrink the pocket down," said Kyle Orton, a Middle East analyst at the Henry Jackson Society think-tank. "Divide it into pieces and empty as many inhabitants as possible.
"But a regime "victory" in Aleppo would just lock in instability - it'd be like Algeria but worse."