UNITED NATIONS - The UN chief has asked the Syrian government for "additional information" on its allegation that the armed opposition used chemical weapons in an attack.

Ban Ki-moon made the request in his reply letter from the Syrian government which requested a UN inquiry into the alleged use of chemical weapons in Syria, which has been plunged into the two-year crisis and subsequent violence, his spokesman Martin Nesirky told reporters here late Thursday.

Ban "requested the Syrian government to provide additional information pertaining to the alleged incident," Nesirky said.

"He also said the mission would require full cooperation from all relevant Syrian authorities and that he counted on the Syrian government's full support, particularly through unfettered access."

Ban, while speaking to reporters in his office here on Thursday morning, said, "I have decided to conduct a United Nations investigation into the possible use of chemical weapons in Syria."

Ban said the investigation will start "as soon as practically possible," adding that his senior advisers are working on the modalities in close consultation with the relevant bodies, including the Organization for the Prohibition of Chemical Weapons (OPCW) and the World Health Organization (WHO).

Detailed issues such as overall mandate, mission composition, and operational conditions, including safety and security, would be considered in those discussions, the secretary-general said.

The UN received a formal request from the Syrian government on Wednesday for a "specialized, impartial and independent mission" to investigate the alleged use of chemical weapons."

At least 25 people were killed and 130 others wounded Tuesday when armed men fired a rocket stuffed with chemical materials at the Khan al-Asal, Syrian state-media said, claiming opposition fighters were responsible. However, the rebels denied the accusations and instead accused the government.

Speaking to reporters at UN headquarters on Wednesday, Bashar Ja'afari, the Syrian permanent UN representative, said he wanted a probe to look into the use of those weapons by "armed terrorist groups" in an attack on the village of Khan al-Assal on the outskirts of Aleppo on March 19.

Ja'afari said the rebel's counterclaim was "to distract attention from the real fact on the ground, which is that the Syrian government, if in fact it has such weapons, would never use them against its own population."

Meanwhile, the secretary-general informed the Syrian government that he had received a separate request from the French and British governments to probe three incidents using the same mechanism provided by General Assembly and Security Council resolutions that is being applied to the Syrian government request, spokesman Nesirky said.

Earlier this week, Ban reiterated that the use of chemical weapons by any side under any circumstances would be an outrageous crime and anyone responsible must be held accountable.