The Syrian military on Tuesday urged rebels to pull out of east Aleppo and allow civilians to also leave, as it pressed an offensive to recapture the city's opposition-held sector.

The army, which has besieged the east for months, also demanded that rebels distribute food to civilians, while calling on residents to cooperate with its advancing troops.

The military airdropped leaflets with a picture of a green coach like those used in the past to transport civilians and rebels from areas retaken by the government.

"To those involved in carrying weapons, we stretch out our hand to you. Reserve your place before it is too late," the leaflets read.

The picture showed the coach on a road marked "the path of salvation".

In a separate statement carried by state media, the army accused rebels of using the 250,000 civilians still in the east as "human shields" and demanded they be allowed to leave.

"Permit those citizens who want to do so to leave, stop using them as hostages and human shields, clear the mines from the crossings identified by the state," the army said.

Syria's government and its ally Moscow have said several crossings are available for civilians and surrendering rebels to cross from east Aleppo, but they accuse opposition fighters of preventing departures.

The statement urged "brother citizens in east Aleppo to cooperate with the Syrian Arab Army", while stressing its "firm commitment to their security and safety".

The army accuses rebels of hoarding food in the east, where aid rations have run out after nearly four months of government siege, and said Tuesday that fighters should distribute rations.

"We call on the gunmen in east Aleppo to open their warehouses and distribute food rations to those in need," the army said.

On Monday, UN humanitarian chief Stephen O'Brien condemned the use of sieges in Syria, saying nearly one million people across the country were under blockades, most imposed by the regime.

"Civilians are being isolated, starved, bombed, denied medical attention and humanitarian assistance in order to force them to submit or flee," he told the UN Security Council.

As the army advances in east Aleppo, its leadership announced the formation of a new voluntary battalion tasked with "eradicating terrorism, fighting alongside the rest of our heroic army and allied factions."

There were no details on who would man the battalion or when and where it would fight.

Syria's army has relied heavily in the fight against rebels on local pro-government militias as well as Lebanon's Shiite militant group Hezbollah and forces from Iran and Russia.

Watchdog accepts Russia offer to aid Aleppo chemical attacks probe
The Hague (AFP) Nov 22, 2016 - A global watchdog probing chemical arms attacks in Aleppo Tuesday took Russia up on an offer to provide some possible evidence saying it "may be of use" to their investigation.

The Russian military said on November 11 that it had evidence of the use of chemical weapons by rebels in Syria's besieged eastern city of Aleppo.

The military said in a statement that its experts "have found unexploded artillery ammunition belonging to terrorists which contains toxic substances."

The Organisation for the Prohibition of Chemical Weapons said Tuesday it had "recently received an offer from Russian authorities to provide some samples and other material in relation to an incident of alleged use of chemicals as weapons in Aleppo."

"These samples and other material may be of use in the ongoing work of the OPCW fact-finding mission," the organisation said in a statement.

It said it had "proposed" to the Russian foreign ministry that given the ongoing conflict in Aleppo, it would be better "to receive such material in Damascus or The Hague."

"At present, the OPCW is awaiting a response," the statement added.

The OPCW, based in The Hague, has launched a fact-finding mission to investigate chemical weapons attacks in Syria.

The mission is feeding its reports to a joint UN-OPCW panel, whose mandate has just been extended for a year.

The OPCW chief, Ahmet Uzumcu, told AFP last week his organisation had received more than 20 reports of chemical attacks since August 1.

On Tuesday, Syrian pro-government forces pushed deeper into eastern Aleppo with the Syrian Observatory for Human Rights saying some residents of the Qaterji and Dahjer Awad districts experienced breathing difficulties after four barrel bombs were dropped which may have contained chlorine gas.