The United Nations will investigate whether chemical weapons have been used in the Syria conflict, UN leader Ban Ki-Moon said Thursday.
Ban said the "difficult mission" would focus on a Syrian government allegation that opposition rebels fired chemical weapons missiles in an attack this week.
But France, Britain and the United States called on Ban to widen the inquiry to include allegations made against President Bashar al-Assad's forces.
The Syrian government has accused opposition rebels of using chemical weapons in Khan al-Assal near Aleppo on Tuesday. The opposition said the government staged the attack and also used banned chemical weapons in another incident near Damascus.
"I have decided to conduct a United Nations investigation into the possible use of chemical weapons in Syria," Ban told reporters in his office. "I intend for this investigation to start as soon as practically possible."
The UN leadership is working with the Organisation for the Prohibition of Chemical Weapons (OPCW), which oversees the Chemical Weapons Convention, and the World Health Organisation to set up the inquiry.
Ban said the mission would "look into the specific incident brought to my attention by the Syrian government."
"I am, of course, aware that there are other allegations of similar cases involving the reported use of chemical weapons," he added, while refusing to take questions on whether the inquiry could be extended.
"Full cooperation from all parties will be essential. I stress that this includes unfettered access," Ban said, calling the investigation "an unequivocal reminder that the use of chemical weapons is a crime against humanity."
"The international community needs full assurance that chemical weapons stockpiles are verifiably safeguarded," Ban said.
Diplomats and UN officials have stressed that there is still no formal proof that chemical weapons have been used by either side.
Britain and France called for a wider inquiry that includes accusations that chemical weapons were also used at Ataybah near Damascus and another incident at Homs on December 23.
"Given the gravity of these allegations, we judge it essential that all the pertinent facts concerning these allegations are swiftly investigated," said a letter sent by the French and British missions to the UN leader. It called for "an urgent investigation into all allegations".
US ambassador Susan Rice said the United States also "supports an investigation that pursues any and all credible allegations of the possible use of chemical weapons in Syria."
US President Barack Obama has said the use of chemical weapons in the conflict could be a "game-changer".
"We demand the full cooperation of the Assad regime in particular, as well as Syrian authorities throughout the country, including by providing full and unfettered access to all relevant individuals and locations," Rice said in a statement.
The Security Council is bitterly divided over the Syria war and Russia has insisted that the inquiry should be limited to the Syrian government allegation.
Russia's UN ambassador Vitaly Churkin welcomed the announcement made by Ban, calling it "a very good, courageous decision."
Russia and China have blocked three western-proposed UN resolutions that would have stepped up pressure on Assad over the conflict in which the UN says well over 70,000 people have died.
Ban said "the horrors" of recent months "prove beyond doubt: the military solution in Syria is leading to the dissolution of Syria."
The OPCW said in a statement released at its base in The Hague that the inquiry would require "careful planning" because of the security dangers.
"While allegations of this nature are not new to conflict situations, they are nonetheless serious, especially in the context of Syria which is not a state party to the Chemical Weapons Convention. This remains a matter of serious concern."