* Government, rebels accuse each other of chemical attack
* UN probe won't focus on 2nd alleged attack in Damascus (Adds quotes, background)
By Michelle Nichols
UNITED NATIONS, March 21 (Reuters) - U.N. Secretary-General Ban Ki-moon on Thursday announced that the United Nations will launch an investigation as requested by the Syrian government into allegations that chemical weapons were used in Syria.
"I have decided to conduct a United Nations investigation into the possible use of chemical weapons in Syria," Ban told reporters. He said the investigation will focus on "the specific incident brought to my attention by the Syrian government."
Syria asked Ban on Wednesday to investigate an alleged chemical weapons attack by "terrorist groups" near the northern city of Aleppo on Tuesday, Syrian U.N. Ambassador Bashar Ja'afari said.
The deaths of 26 people in a rocket attack on the northern town of Aleppo on Tuesday have become the focus of a propaganda war between Syrian President Bashar al-Assad's supporters and opponents, who accuse each other of firing a missile laden with chemicals.
The Syrian opposition said on Wednesday that there was a second chemical weapons attack on Tuesday in Damascus in addition to the one the government and opposition accuse each other of carrying out in Aleppo on the same day.
British and French U.N. envoys said after a closed-door meeting of the Security Council on Wednesday that they wanted the United Nations to investigate both the alleged attacks. Russia said the Western request was an attempt to delay a U.N. investigation, an accusation the Europeans rejected.
Ban made clear that the focus of the investigation he announced would be the Aleppo attack.
"I am of course aware that there are other allegations of similar cases involving the reported use of chemical weapons," he said, adding that the United Nations would be cooperating with the Organisation for the Prohibition of Chemical Weapons (OPCW) and the World Health Organisation (WHO).
"Full cooperation from all parties will be essential. I stress that this includes unfettered access," he said. "I reiterated this point in my communications with the Syrian authorities."
"There is much work to do and this will not happen overnight, it is obviously a difficult mission," Ban said. "I intend for this investigation to start as soon as is practically possible."
Ban said his announcement "should serve as 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," he said.
U.S. and European officials say there is no evidence to suggest there was a chemical weapons attack. If confirmed, it would be the first use of such weapons in the two-year Syrian conflict, which the U.N. says has killed 70,000 people. (Additional reporting and writing by Louis Charbonneau; Editing by Vicki Allen)