President Barack Obama has decided to authorise lethal aid to Syrian rebels but US officials are still grappling with what type and how much weaponry to send the opposition forces and how to ensure it stays out of the hands of extremists battling for control of Syria.

The White House announced it had conclusive evidence that Syrian President Bashar Assad's regime used chemical weapons against opposition forces. Mr Obama has said the use of chemical weapons would cross a "red line," suggesting greater American intervention.

While a small percentage of the 93,000 people reportedly killed in Syria are said to have died from chemical weapons - US intelligence puts the number at 100 to 150 - the White House views the deployment of the deadly agents as a flouting of international norms. Ben Rhodes, Obama's deputy national security adviser, said the multiple chemical weapons attacks gave greater urgency to the situation.

"Suffice it to say this is going to be different in both scope and scale in terms of what we are providing," Mr Rhodes said . But US would make specific determinations "on our own timeline."

The Obama administration could give the rebels a range of weapons, including small arms, assault rifles, shoulder-fired remote-propelled grenades and other anti-tank missiles. The opposition forces could operate most of that equipment without significant training.

Mr Obama's opposition to sending American troops into Syria makes it less likely the US will provide sophisticated arms or anti-aircraft weapons that would require large-scale training. Administration officials are also worried about high-powered weapons ending up in the hands of terrorist groups. Hezbollah fighters are among those backing Assad's armed forces, and al Qaida-linked extremists back the rebellion.

The CIA and special operations trainers are already running some weapons training programmes for the rebels and are expected to take charge of teaching the opposition how to use the weapons the US has agreed to supply, another US official said.

There is also some debate within the administration about who would provide the lethal aid and how it might be delivered, the US officials said.

Mr Obama has resisted arming the rebels until now, a cautious approach that underscores the deep divisions within his administration. The proponents of more aggressive action, including Secretary of State John Kerry, appeared to have won out over those wary of sending weapons and ammunition into the war zone.

The US has made no decision on operating a no-fly zone over Syria, Mr Rhodes said.

The US has so far provided the Syrian rebel army with rations and medical supplies. The administration has also agreed in principle to provide body armor and other equipment such as night-vision goggles to the rebels, although the Pentagon has said there has been no movement on that as yet.

The new move followed new US intelligence assessments showing that Mr Assad has used chemical weapons, including sarin, on a small scale multiple times in the last year, killing an estimated 100 to 150 people.

Mr Obama advisers believe Mr Assad's regime still maintains control of Syria's chemical weapons stockpiles and does not see any evidence that rebel forces have launched attacks using the deadly agents.

The administration announced in April that it had "varying degrees of confidence" that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.

The more conclusive findings announced last night were aided by evidence sent to the United States by France, which, along with Britain, has announced it had determined that Mr Assad's government had used chemical weapons.

In Brussels, Nato Secretary-General Anders Fogh Rasmussen said today: "The international community has made clear that any use of chemical weapons is completely unacceptable and a clear breach of international law."

He said he welcomes the "clear US statement" and called on Syria to "grant access to the United Nations to investigate all reports of chemical weapons use."

Mr Obama will discuss the chemical weapons assessments, along with broader problems in Syria, during the summit of eight leading industrial nations next week in Northern Ireland.

Among those in attendance will be Russian President Vladimir Putin, one of Mr Assad's most powerful backers. Today Mr Putin's foreign affairs adviser said Moscow was not convinced with Washington's claims.

Yuri Ushakov told reporters that the information provided by US officials to Russia "didn't look convincing."

The Syrian fighters have been clamouring for bolder Western intervention, particularly given the estimated 5,000 Hezbollah guerrillas propping up Mr Assad's forces. Mr Assad's stunning military success last week at Qusair, near the Lebanese border, and preparations for offensives against Homs and Aleppo have made the matter more urgent.

Ed Royce, a Republican congressman who is the chairman of the House Foreign Affairs Committee, said he supported the president's decision "to expand assistance for the vetted Syrian opposition." But other politicians expressed reservations, including Sen. Chris Murphy, a Democrat, and a member of the Senate Foreign Relations Committee.

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