Syrian regime forces retook almost a third of the Masaken Hanano district on Tuesday, a Britain-based monitoring group has said.

"This is a key and important advance for the Syrian army since the assault on eastern Aleppo started," Rami Abdel-Rahman, head of the Syrian Observatory for Human Rights, told the German news agency dpa.

Activists inside Aleppo told Agence France-Presse that the last few residents of Masaken Hanano - which has been targeted regularly by air strikes - were fleeing to other areas inside eastern Aleppo.

Rebel leader killed

During street battles in the nearby Sheikh Saeed neighborhood, the head of the hard-line Islamist rebel group Ahrar al-Sham was killed, Adbel-Rahman added.

Ahrar al-Sham is thought to be the largest rebel force in northern Syria.

The Observatory said at least 141 civilians, including 18 children, have been killed over the past week, as regime troops - with support from Russian and Syrian air strikes - stepped up their bombardment on the rebel-held eastern enclave of Aleppo.

More than 25 people died over the past 24 hours, Abdel-Rahman said.

The war monitor said it had documented hundreds of injuries since October 15, and that hospitals in the area had been left devastated the latest onslaught.

Air strikes by Russian and Syrian forces are assisting the ground advance on eastern Aleppo

A further 87 deaths - either rebel fighters or people of unknown identity had been reported in Aleppo's eastern sector. Rebel shelling had also caused more than a dozen civilian deaths and scores of injuries, the Observatory said.

Humanitarian situation worsening

As many as 300,000 people remain under siege in eastern Aleppo, amid reports of declining food, water, medical and fuel supplies.

On Tuesday, the Syrian army urged rebels to distribute tons of food being kept in local warehouses to feed people, claiming there were still enough stocks to feed those trapped civilians.

The military once again urged rebels to pull out of the areas of the city they control.

Washington and the United Nations have condemned the latest government advance, which if successful would be a major strategic victory for President Bashar al-Assad.

Syrian rebel groups captured the eastern side of what was Syria's largest city in terms of population.

Once the country's economic powerhouse, Aleppo has seen widespread destruction and has been sealed off from the outside world since the army surrounded it in mid-July.

Russia hits back

Also on Tuesday, Russian Foreign Minister Sergei Lavrov on Tuesday accused the UN Syria envoy Staffan de Mistura of "sabotaging" a resolution backing peace talks between the regime and opposition.

The most recent UN Security Council Resolution, adopted in December 2015, proposed the establishment of a body to head a political transition for the war-ravaged country.

But it has been plagued by disagreements over the fate Assad, whom the Syrian opposition does not want to see in any future role.

UN-brokered talks with opposition groups have broken down several times during the year.

mm/kl (AFP, dpa, Reuters)