The Syrian army and its allies made sweeping advances in Aleppo on Monday, raining fire on rebels and pushing them to the brink of collapse in a shrinking enclave packed with civilians who said they faced "doomsday in Aleppo".
"The bombardment did not stop for a moment overnight," said a Reuters journalist in the government-held zone of the city, describing it as the most intense for days.
Pro-government forces were clashing with insurgents in Fardous district, which was at the heart of the besieged pocket only days ago, after taking Sheikh Saeed in the south and Saliheen in the east, a rebel official said.
"The situation is extremely difficult today," said Zakaria Malahifji of the Fastaqim rebel group fighting in Aleppo.
The rebels' sudden retreat represented a "big collapse in terrorist morale", a Syrian military source said.
Syrian President Bashar al-Assad is now close to taking back full control of Aleppo, which was Syria`s most populous city before the war and would be his greatest prize so far after nearly six years of conflict.
Rebel groups in Aleppo received a US-Russian proposal on Sunday for a withdrawal of fighters and civilians from the city's opposition areas, but Moscow said no agreement had been reached yet in talks in Geneva to end the crisis peacefully.
"People run from one shelling to another to escape death and just to save their souls... It`s doomsday in Aleppo, yes doomsday in Aleppo," said Abu Amer Iqab, a former government employee in the Sukkari district in the heart of the rebel enclave.
The rebel official blamed Russia for the lack of progress in talks, saying it had no incentive to compromise while its ally Assad was gaining ground. "The Russians are being evasive. They are looking at the military situation. Now they are advancing," he said.
While Alepp's fall would deal a stunning blow to rebels trying to remove Assad from power, he would still be far from restoring control across Syria. Swathes of the country remain in rebel hands, and on Sunday Islamic State retook Palmyra.
Tens of thousands of civilians remain in rebel-held areas, hemmed in by ever-changing front lines, pounded by air strikes and shelling, and without basic supplies, according to the Syrian Observatory for Human Rights, a British-based monitoring group.
The Observatory said the Sheikh Saeed district had fallen to the army in fighting on Sunday night and early on Monday and troops were firing on the districts of Karam al-Daadaa and Fardous.
An advance into those districts would take the army into the heart of the area held by rebels as recently as Saturday, pushing them towards a last bastion of control on the west bank of Aleppo`s river and the area southwest of the citadel.
The Syrian army is backed by Russian war planes and Shi`ite militias supported by Iran. The mostly Sunni rebels include groups backed by the United States, Turkey and Gulf monarchies as well as hardline jihadists who are not supported by the West.
A correspondent for Syria`s official SANA news agency said the army had taken control of Sheikh Saeed, and more than 3,500 people had left at dawn.
A Syrian official told Reuters: "We managed to take full control of the Sheikh Saeed district. This area is very important because it facilitates access to al-Amariya and allows us to secure a greater part of the Aleppo-Ramousah road." The road is the main entry point to the city from the south.
The loss of Palmyra, an ancient desert city whose recapture from Islamic State in March was heralded by Damascus and Moscow as vindicating Russia`s entry into the war, is an embarrassing setback to Assad.
The Observatory reported that the jihadist group carried out eight executions of Syrian soldiers and allied militiamen in Palmyra on Monday while warplanes bombarded their positions around the city.
Another four people, including two children, were shot dead while the jihadists cleared the city of pro-government forces, it said.
The Russian Defence Ministry said on Monday that 728 rebels had laid down their weapons over the previous 24 hours and relocated to western Aleppo. It said 13,346 civilians left rebel-controlled districts of Aleppo over the same period.
The Observatory said that four weeks into the army offensive at least 415 civilians, including 47 children, had been killed in rebel-held parts of the city.
Hundreds had been injured by Russian and Syrian air strikes and shelling by government forces and its allies on the besieged eastern part of the city.
The Observatory said 364 rebel fighters had been killed in the eastern sector. It said rebel shelling of government-held west Aleppo had killed 130 civilians, including 40 children. Dozens had been injured.
The army on Sunday took foreign journalists to witness an enlistment ceremony for 220 men, including former rebels and others from opposition-held areas captured by the government. The fate of young men leaving the shrinking rebel pocket in Aleppo has been a subject of argument between the two sides.
Opposition supporters have accused the government of mass arrests and extrajudicial killings, which Damascus has denied. The government accuses rebels of forcing people to fight for them and preventing them leaving, which the insurgents deny.
The United Nations said last week it was concerned about reports that hundreds of young men had been detained upon leaving the rebel-held enclave.