It increasingly appears that a chemical weapon was not used in Syria this week, a US official says, although officials cautioned that US intelligence agencies have not yet reached a final conclusion.

"Our growing sense is that weaponised CW was not used," the official said, speaking on condition of anonymity. Still, the official left open the possibility that information could arise that changed the analysis.

Syrian President Bashar al-Assad's government and rebels accused each other of using chemical weapons in a rocket attack near Aleppo on Tuesday that killed 26 people.

A European security official said that if chemical weapons or other "weapons of mass destruction" had been fired off, the casualty toll would be much higher than 26.

The official said he did not believe that the evidence showed chemical weapons had been used.

After the attack on Tuesday (local time), some of those hospitalised told a Reuters photographer they detected a strong smell of chlorine in the air and that many victims had fallen down dead after the blast.

President Barack Obama said in Israel on Wednesday that Assad would be held accountable if it were determined that chemical weapons had been used in Syria.

On Thursday a US intelligence official said, "the intelligence community has not made an assessment as to whether or not chemical weapons were used or not".

US Ambassador to the United Nations Susan Rice issued a statement on Thursday welcoming the announcement of a UN investigation.

"As the UN proceeds with these efforts, we will also continue to work closely with our partners to obtain further information regarding any and all credible allegations of the potential or actual use of chemical weapons in Syria," she said.

Two senators wrote a letter to Obama saying that more must be done to stop the killing in Syria and force Assad to relinquish power.

"The potential use of chemical weapons only makes the case for greater action more compelling and urgent," said the letter from Senate Armed Services Committee Chairman Carl Levin, a Democrat from Michigan, and Republican Senator John McCain of Arizona.

UN INVESTIGATING

The United Nations says it will 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 told reporters.

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

Ban made clear the focus of the investigation he announced would be on 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 co-operating with the Organisation for the Prohibition of Chemical Weapons and the World Health Organisation.

"My announcement should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity," Ban said.

US Ambassador to the United Nations Susan Rice said Washington wanted any serious allegations regarding the use of chemical weapons in Syria to be investigated.

"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.

DEADLOCK

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."

US 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 two-year Syrian conflict, which the United Nations says has killed 70,000 people.

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

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

The dispute over the scope of the UN 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.

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.

Israel, Myanmar, Angola, Egypt, North Korea, Somalia and South Sudan are the only other states who have not joined the 1997 convention.

This is the first time the Organisation for the Prohibition of Chemical Weapons, which oversees the convention, has been asked to investigate in a conflict zone. A team of inspectors is ready to travel to Syria when it is safe to do so.

The World Health Organisation said it would support the investigation as requested by Ban. "The overall mandate, mission composition and operational conditions, including safety and security, have yet to be finalized. No start date has been set," said WHO spokesman Gregory Hartl.

Ban urged the Syrian government and rebels to co-operate.

"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.

- Reuters