Government aircraft dropped TNT-packed barrels on a crowded vegetable market in a rebel-held neighborhood of Aleppo on Saturday, killing at least 25 people, the Syrian Observatory for Human Rights reported.

"The number of people killed has risen to 25 including two women, four children, a teenager and a media activist," the Britain-based watchdog said of the raid, which destroyed numerous buildings, including part of a hospital.

"The number is likely to rise further because several people have been critically injured," the Observatory added.

Videos posted online by local activist group Insaan Rights Watch show residents pulling mangled bodies from smashed cars. Another amateur video showed a road lined with bodies covered in blankets. The content of the videos could not be independently verified.

The pro-opposition Syrian Observatory also claims more than 400 people were killed in a massive aerial offensive on rebel-held areas of Aleppo and nearby villages launched by forces loyal to Syrian President Bashar al-Assad on December 15.

'Indiscriminate' bombardment

The alleged use of the so-called "barrel bombs" in the latest government campaign has drawn international criticism from human rights groups as well as Arab and Western countries. They say the bombs - oil drums or cylinders packed with explosives and metal fragments - are an indiscriminate form of bombardment.

The Assad regime asserts the offensive is targeting "terrorists."

Observers suggest the assault on Aleppo is designed to expose the opposition's weakness in order to strengthen President Assad's position ahead of planned peace negotiations in Geneva next month.

Aleppo, Syria's largest city and former business hub, has been heavily damaged since rebels launched an offensive there in mid-2012. It has since been divided into rebel and government-held areas.

Chemical weapons setback

Meanwhile the United Nations acknowledged for the first time on Saturday that Syria is "unlikely" to meet a December 31 deadline to transport its most dangerous chemical arms out of the country.

In a joint statement the Organization for the Prohibition of Chemical Weapons (OPCW) and the UN said "important progress" had been made in dealing with Syria's chemical weapons stockpile, however the Assad government needed to "intensify efforts" to meet internationally set deadlines.

"Preparations continue in readiness for the transport of most of the critical chemical material from the Syrian Arab Republic for outside destruction. However, at this stage, transportation of the most critical chemical material before 31 December is unlikely," the statement said.

The two bodies blamed external factors, primarily security concerns, bad weather and logistical problems for the delay.

The end-of-year deadline was the first to be set as part of a US-Russia brokered deal to eliminate Syria's chemical weapons by the middle of 2014. The deal requires that the weapons be taken to a port in Italy where they would be transported to a US Navy ship capable of destroying them at sea.

The deal averted the imminent threat of US military strikes aimed at the Syrian regime after US officials said they had evidence some 1,400 people were killed in a chemical weapons attack near Damascus in August.

According to UN estimates more than 100,000 people have been killed across the country since the uprising against the Assad regime began in March 2011. Hundreds of thousands of refugees have fled to neighboring countries including Lebanon, Turkey and Jordan.

ccp/pfd (AFP, AP, Reuters)