UNITED NATIONS/BEIRUT: The United Nations said Thursday it would investigate Syria’s allegations that rebel forces used chemical weapons in an attack near Aleppo, but Western countries sought a probe of all claims concerning the use of such banned arms.

“I have decided to conduct a United Nations investigation into the possible use of chemical weapons in Syria,” said Secretary-General Ban Ki-moon.

The investigation will focus on “the specific incident brought to my attention by the Syrian government,” he said.

Syria asked Ban to investigate an alleged chemical weapons attack by “terrorist groups” near the northern city of Aleppo Tuesday, said Syria’s U.N. Ambassador Bashar Jaafari.

The deaths of at least 25 people in that rocket attack have become the focus of competing claims from Syrian President Bashar Assad’s supporters and opponents, who accuse one another of firing a missile laden with chemicals.

The Syrian opposition reported a second chemical weapons attack Tuesday near Damascus.

Ban made clear the focus of the investigation he announced would be on the Aleppo attack.

“I am of course aware there are other allegations of similar cases involving the reported use of chemical weapons,” he said, adding that the U.N. would be cooperating with the Organization for the Prohibition of Chemical Weapons and the World Health Organization.

U.S. Ambassador to the U.N. Susan Rice said Washington wanted any serious allegations regarding the use of chemical arms in Syria to be probed.

“The United States supports an investigation that pursues any and all credible allegations of the possible use of chemical weapons in Syria, and underscores the importance of launching this investigation as swiftly as possible,” Rice said in a statement.

France and Britain wrote to Ban on Thursday to draw his attention to the second alleged attack near Damascus, as well as one in Homs in late December. The rebels blame Syria’s government for all three incidents

“Given the gravity of these allegations we judge it essential that all the pertinent facts concerning these allegations are swiftly investigated,” France and Britain wrote. “We therefore request that you launch an urgent investigation into all allegations as expeditiously as possible.”

U.N. spokesman Martin Nesirky said Ban was reviewing the British and French request. It was not clear whether the Syrian government’s permission would be required to broaden the investigation to include all three alleged attacks. One U.N. diplomat said Assad’s government would have to consent to it.

Russian Ambassador Vitaly Churkin said Wednesday the British and French demand for an investigation into other attacks was an attempt to delay the U.N. probe of the Aleppo incident. He praised Ban’s decision to begin a probe.

The dispute over the scope of the U.N. investigation highlights the chasm between Russia’s position toward the Syrian government, its ally, and that of the Western powers who support the opposition trying to oust Assad. The deadlock on the council has left it powerless to act on Syria.

U.S. and European officials say there is no evidence of a chemical weapons attack. If one is confirmed, it would be the first use of such weapons in the 2-year-old Syrian conflict, which the U.N. says has killed 70,000 people.

Syria is not a party to the Chemical Weapons Convention, an international treaty that bans chemical weapons. For months the United States, Israel and European countries have voiced concerns about the security of Syria’s chemical arms stockpile.

Ban urged the Syrian government and rebels to cooperate.

“There is much work to do and this will not happen overnight, it is obviously a difficult mission,” Ban said, adding the probe will begin as soon as possible.

He said the investigation “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.”

Elsewhere, Syrian opposition leader Soheir Atassi of the Syrian National Coalition grouping said Thursday she was rejoining the body a day after abruptly “freezing” her membership.

“The main reason I froze my participation is the failure of organizational work and the lack of professionalism,” Atassi wrote on her Facebook page.

“I canceled my decision ... [and] I announce that I am resuming all my responsibilities ... after serious and in-depth discussions suggesting a significant change that I demanded,” Atassi wrote.

Atassi’s decision to freeze her membership in the coalition came shortly after the body elected Ghassan Hitto to serve as Syria’s first rebel premier, tasking him with forming an interim government.

Hitto’s election, and the move to form a government, were opposed by some coalition members, around 10 of whom joined Atassi in announcing they were suspending their membership.

But Atassi backs the forming of an interim government and emphasised on her Facebook page Thursday that she continued to support the process “as an institutional way of serving the revolution.”