The United States has signalled it is preparing to insert itself directly in the Syrian civil war by for the first time giving direct military support, including arms, to moderate rebels fighting the regime of President Bashar al-Assad.

The move, confirmed by a senior foreign policy official, coincided with a formal determination by the White House, also for the first time, that President al-Assad has used chemical weapons in the two-year-old struggle. It was that assessment, they said, that persuaded President Barack Obama to offer the rebels military hardware.

“The president has said that the use of chemical weapons would change his calculus, and it has,” White House deputy national security adviser Benjamin Rhodes told reporters in a conference call on Thursday evening. He said that US intelligence estimates that attacks using the chemicals had killed between 100 and 150 people.

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Debate has been raging for weeks inside the White House over the worsening Syrian conflict which is now believed to have taken more than 90,000 lives. President Obama believes that the American public is not eager to see the country drawn into another war, even in a limited way, so soon on the heels of those in Iraq and Afghanistan. But he has been saying for months that the use of chemical weapons would be a trigger for American action.

“Following a deliberative review, 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,” the White House said in a statement. By accepting that President al-Assad has used chemical weapons, the US is catching up with the European intelligence community which had already reached that conclusion.

The US made the announcements one day after a visit to Washington by the Foreign Secretary, William Hague, whose meeting with John Kerry, the US Secretary of State, was dominated by the Syrian crisis. The topic is now certain to eclipse other business at next week’s G8 summit to be hosted by the Prime Minister, David Cameron, in Northern Ireland. Discussions there will be key because Russia, Syria’s biggest defender, will be attending.

Mr Rhodes gave scant detail of what it is that the US means to supply the rebels. But just this week the head of the opposition Supreme Military Council, or SMC, made an impassioned appeal to the US and other nations for arms as its main base in the city of Aleppo has been seen as increasingly threatened by government forces.

But that the US will be supplying lethal arms no longer seems to be in question. “Suffice it to say, this is going to be different in both scope and scale in terms of what we are providing to the SMC than what we have provided before,” Mr Rhodes said. It is likely that the CIA will be tasked with determining what should be shipped to the Council and how it will be logistically set up.

Meanwhile there were reports that contingency plans drawn up by the US military for consideration by the White House include the imposition of a limited no-fly zone in parts of Syria close to Jordan. The zone’s purpose would be to shield an area inside Jordan that would be created as a base to train and arm the Syrian rebels.

Mr Rhodes however declined to comment on any such development. The president, he insisted, has “not made any decision to pursue a military option such as a no-fly zone.” But he made clear that the decision to arm the rebels had been borne from deepening concern about the conflict and the threat it poses to regional stability.

“There is an urgency to the situation,” he commented. “There has been an urgency to the situation for two years. “It’s particularly urgent right now in terms of the situation on the ground, in some respect, because we have seen Hezbollah and Iran increase their own involvement in the conflict.”