Syrian warplanes killed 27 civilians in a raid on a village market Thursday, as President Bashar al-Assad's regime nearly completed the surrender of its chemical weapons stockpile.

The air raid on the Aleppo provincial village of Atareb, where the Syrian Observatory for Human Rights said three children were among the dead, came amid a massive aerial offensive on Aleppo's rebel areas that began in mid-December.

The campaign has killed hundreds, mostly civilians, and forced thousands of families to flee.

Aleppo-based activist Abu Omar said that a market area was hit -- "that's why there were so many civilians killed.

"The regime is hitting back against the civilians who support the revolt" against Assad, he added.

Activists distributed amateur video footage showing chaotic scenes, with bodies lying among mounds of rubble.

One video showed a woman in a white headscarf screaming as she leans over the body of a loved one.

It also showed a man attending a boy whose leg has been ripped off. It was unclear whether the child was alive or dead.

- Chemical handover nearly complete -

The joint Organisation for the Prohibition of Chemical Weapons-UN task team charged with overseeing the destruction of Syria's chemical arsenal said 92.5 percent of the stockpile has been surrendered.

Damascus had pledged to have all of its stockpile removed from Syria by Sunday, with the weapons due to be destroyed by June 30.

On Wednesday, UN Security Council members called for a probe into new claims of a chlorine gas attack in a rebel bastion.

Nigerian Ambassador Joy Ogwu, who holds the council's rotating presidency, said there was concern over reports about chlorine gas killing and injuring several people, and called for an investigation.

There have been conflicting accounts of an alleged chlorine attack on opposition-held Kafr Zita, with the government and the opposition trading blame.

Under the terms of a US-Russian brokered deal that averted the threat of US military action last year, Syria agreed to destroy its chemical stockpiles.

The deal was reached after deadly chemical attacks outside Damascus last August killed hundreds of people.

While the final destruction of the stockpile appears imminent, analysts were raising the matter of production sites.

Sico van der Meer of the Clingendael Institute said "they will complete the removal, but the question of production sites is still there".

Damascus wants to seal the sites, which it says have been rendered unusable, but Western countries want them completely destroyed, fearing they could be re-activated.

"Syria is playing for time. As long as the process of destroying its chemical weapons is under way, the international community is not going to bother it too much," Van der Meer said.

Despite the violence, the regime plans a presidential election on June 3 that is expected to return Assad to office.

On Thursday, regime-tolerated opponent Hassan Abdullah al-Nuri became the second candidate to register his candidacy, a day after independent MP Maher al-Hajjar did so.

- Food aid distributed -

Meanwhile, 22 rebel and jihadist Al-Nusra Front fighters were killed in a battle in Daraa province, as they seized from army control a strategic hilltop overlooking besieged Nawa town, said the Observatory.

The group also said an unknown number of regime troops were taken prisoner or killed.

Abu Anas, an activist in the southern province, told AFP: "The army had been using the hilltop to shell many parts of Daraa."

In southern Damascus, the UN was allowed to distribute 300 parcels of food aid in besieged Yarmuk, after a 15-day hiatus, UN Relief and Works Agency spokesman Chris Gunness said.

More than 100 people have died in the past year from food and medical shortages in Yarmuk, a Palestinian refugee camp.

However, UN Secretary General Ban Ki-moon said access for foreign aid for millions of Syrians desperately in need has not improved overall, violating a Security Council resolution passed two months ago.

Almost 3.5 million people remain without access to essential goods and services, including life-saving medicines, in a "clear violation" of international law, said Ban.

Syria's conflict is estimated to have killed more than 150,000 people, with millions having fled their homes.

In other developments, Belgian Interior Minister Joelle Milquet called for closer cooperation with European partners and others to stop young fighters joining rebels in Syria.

"Coping with the return (of fighters), that is our main concern," Milquet said, pointing to the presence of Al-Qaeda-linked groups in Syria.