Aleppo (Syria) (AFP) - Hundreds of civilians have fled rebel-held east Aleppo after government forces, determined to retake all of Syria's second city, seized its largest opposition-controlled district.
The capture on Saturday of Masaken Hanano -- which had been the biggest rebel-held district of Aleppo -- was a major breakthrough in a 13-day regime offensive to retake the entire city.
The fighting moved to two neighboring districts, Haidariya and Sakhur, on Sunday, with regime aircraft pounding rebel positions and heavy fighting between the opposition and forces loyal to President Bashar al-Assad.
Masaken Hanano was the first district the rebels took in the summer of 2012 in a move that divided the city into a rebel-held east and a regime-controlled west.
Around 250,000 civilians trapped under government siege for months in the east have faced serious food and fuel shortages.
More than 500 civilians fled rebel-held districts for the government-controlled west overnight, the Syrian Observatory for Human Rights monitoring group said on Sunday.
The civilians fled to Masaken Hanano after it fell under government control and were taken by the army to regime-held areas, the Observatory said.
"It is the first exodus of this kind from east Aleppo since 2012," Observatory chief Rami Abdel Rahman said.
'Aircraft destroying everything'
Yasser al-Youssef, from the rebel group Nureddin al-Zinki, said on Sunday that opposition fighters were consolidating their positions in Sakhur.
"We are strengthening our positions to defend the city and residents, but the aircraft are destroying everything methodically, area by area," he said, referring to a regime campaign of air strikes on the city.
Sakhur lies on a stretch of just 1.5 kilometres (less than a mile) between west Aleppo and Masaken Hanano, now both controlled by the regime.
If the regime did manage to take control of the district, east Aleppo would be split in two from north to south, dealing a further blow to the armed opposition.
Pro-government media reported government forces continued their advance on Sunday.
The latest regime push comes after days of intense bombardment on the east, which has been pounded with air strikes, shells and barrel bombs.
On Saturday, dozens of families fled Sakhur and Haidariya as regime raids and artillery fire killed at least 18 civilians in several districts, the Britain-based Observatory said.
That took to 219 the overall number of civilians killed, including 27 children, since the government launched its latest assault on east Aleppo on November 15.
IS chemical attack
Rebel forces also intensified rocket attacks on western districts overnight, killing at least four civilians and wounding dozens, the Observatory said.
Such attacks have killed a total of 27 civilians since the offensive began, among them 11 children.
The United Nations has a plan to deliver aid to Aleppo and evacuate the sick and wounded, which rebel factions have approved but which Damascus has not yet agreed. Guarantees are also needed from regime ally Russia.
Once a commercial and industrial hub, Aleppo has seen some of the worst fighting in Syria's five-and-a-half-year war.
The conflict broke out in 2011 with the brutal repression of anti-government protests and has since evolved into a complex war involving different factions and foreign powers.
On Sunday, the Turkish army said that 22 pro-Ankara Syrian rebels were hit by a chemical gas attack from Islamic State group jihadists in northern Syria.
The Turkish army is backing the Syrian fighters in an unprecedented cross-border operation it says is targeting both IS and the Kurdish People's Protection Units (YPG) militia, which it considers to be a "terrorist" group.
The YPG is a key component of a US-backed Arab-Kurdish alliance that is fighting to oust IS from its de facto Syria capital of Raqa, after the jihadist group overran large parts of Syria and Iraq in 2014.
Syria's war has killed more than 300,000 people and displaced more than half the population.
SEE ALSO: This is what Aleppo is