WASHINGTON/BEIRUT (REUTERS) - The United States has concluded that Syrian President Bashar al-Assad's forces used chemical weapons against rebel fighters and Washington will supply direct military assistance to the opposition, the White House said on Thursday.

The new assessment and decision came as Mr Assad's surging forces and their Lebanese Hezbollah allies turned their guns on the north, fighting near the northern city of Aleppo and bombarding the central city of Homs after having seized the initiative by winning the open backing of Hezbollah last month and capturing the strategic town of Qusair last week.

With outgunned rebel forces desperate for weapons after their battlefield setbacks, US President Barack Obama's deputy national security adviser, Ben Rhodes, said on Thursday that the president had decided to provide "direct military support" to the opposition.

But he would not specify whether the support would include lethal aid, such as weapons, which would mark a reversal of Mr Obama's resistance to arming the rebels.

The announcement followed deliberations between Mr Obama and his national security aides as pressure mounted at home and abroad for more forceful action on the Syria conflict, including a sharp critique from former President Bill Clinton.

The arrival of thousands of seasoned, Iran-backed Hezbollah Shi'ite fighters to help Mr Assad combat the mainly Sunni rebellion has shifted momentum in the two-year-old war, which the United Nations said on Thursday had killed at least 93,000 people.

US and European officials anxious about the rapid change are meeting the commander of the main rebel fighting force, the Free Syrian Army (FSA), on Friday in Turkey. FSA chief Salim Idriss is expected to plead urgently for more help.

Mr Obama has so far been more cautious than Britain and France, which already forced the European Union this month to lift an embargo that had blocked weapons for the rebels.

After months of investigation, the White House laid out its conclusions on chemical weapons use by Mr Assad's forces but stopped short of threatening specific actions in response to what Mr Obama said would be a "game changer" for Washington's handling of the conflict.

"Our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," Mr Rhodes told reporters.

"Our intelligence community has high confidence in that assessment given multiple, independent streams of information," he said. "The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete."

Mr Rhodes said the US military assistance to the rebels would be different in "both scope and scale" from what had been authorised before, which included non-lethal equipment such as night-vision goggles and body armor.

Pressed on what the United States would do next, Mr Rhodes said the White House would share the information with Congress and US allies but will "make decisions on our own time line".

Syrian rebel and political opposition leaders immediately called for anti-aircraft and other sophisticated weapons.

Western governments that predicted months ago that Mr Assad would soon fall now believe that support from Teheran and Hezbollah are giving him the upper hand. But they also worry that sending arms to rebel fighters could empower Sunni Islamist insurgents who have pledged their loyalty to Al-Qaeda.

While Britain and France have yet to announce their own decisions to start arming the rebels, their diplomats have been making the case that the best way to counter both threats is to beef up support for Mr Idriss' mainstream rebel force.

Strengthening the FSA with money, weapons and ammunition, they argue, would help combat Mr Assad and also provide a counterweight among the rebels to Al-Qaeda-linked groups.

France in particular has developed good relations with Idriss while providing funds and non-lethal support, and seems eager to send him military aid.

Among those whose comments put pressure on Obama to act was one of his predecessors, Bill Clinton.

"The only question is: now that the Russians, the Iranians and Hezbollah are in there head over heels... should we try to do something to try to slow their gains and rebalance the power so that these rebel groups have a decent chance to prevail," the former president was quoted by newspaper Politico as saying.

Mr Assad's government says its next move will be to recapture Aleppo in the north, Syria's biggest city and commercial hub, which has been divided since last year when advancing rebels seized most of the countryside around it.

Leading Sunni Muslim clerics met in Cairo on Thursday and issued a call to jihad against Mr Assad and his allies on Thursday, condemning the conflict as a "war on Islam."