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The White House bumped up its confidence Thursday that Syria鈥檚 Bashar al-Assad has used chemical weapons against his own people, but it stopped short of issuing a definitive conclusion on what President Obama has said would be a 鈥済ame-changer鈥� for US policy on Syria.

In a letter to two senators, the White House said US intelligence agencies have assessed 鈥渨ith varying degrees of confidence鈥� that the Assad regime has used chemical weapons on a small scale in the country鈥檚 civil war.

The White House says 鈥減hysiological samples鈥� led to an assessment that the chemical agent sarin was used. But it also cautions that the United States is not certain about the 鈥渃hain of custody鈥� of the samples that were evaluated, and 鈥渟o we cannot confirm how the exposure occurred and under what conditions.鈥�

The letter adds that 鈥渋ntelligence assessments alone are not sufficient鈥� and that the US would only act on 鈥渃redible and corroborated facts that provide us with some degree of certainty鈥� about the use of chemical weapons.

Since last August, Mr. Obama has laid down use or transfer of Syria鈥檚 substantial chemicals-weapons stockpile as a 鈥渞ed line,鈥� the crossing of which would trigger a serious US response. The White House letter puts the US closer to acknowledging that the 鈥渞ed line鈥� has been crossed, while maintaining that more facts are needed before the US has the level of certainty required to act.

One of the senators receiving Thursday鈥檚 letter, Sen. John McCain (R) of Arizona quickly cast a negative light on the White House position, suggesting to journalists that it was formulated in a manner to allow the president to put off the kind of intervention in Syria that he and other members of Congress favor. Senator McCain told CNN that the letter was crafted to give the president 鈥渁n out.鈥�

The letter, which was signed by White House legislative affairs director Miguel Rodriguez, was also sent to Sen. Carl Levin (D) of Michigan, chairman of the Senate Armed Services Committee.

The White House position comes amid a flurry of varying assertions and assessments of reported cases of chemical weapons being used in Syria this year. This week a senior Israeli military intelligence officer declared that Mr. Assad鈥檚 forces have used chemical weapons on a number of occasions this year. This came after Britain and France sent a letter to United Nations Secretary-General Ban Ki-moon underscoring the urgency of investigating cases where evidence pointed to some use of chemical weapons.

A senior French official speaking on condition of anonymity Thursday said France has 鈥渘o national evidence鈥� proving use of chemical weapons 鈥� suggesting that the evidence coming out of Syria could be from interested parties and could not been independently verified.

Since shortly after a reported March 19 attack with chemical weapons on communities outside Aleppo and Damascus, some British officials have said they obtained soil samples from at least one of the sites.

On Thursday, Defense Secretary Chuck Hagel said in comments in Abu Dhabi, capital of the United Arab Emirates, that the US now suspects that Assad has used chemical weapons on a 鈥渟mall scale.鈥� But he went on to say that the finding was based on 鈥渧arying degrees of confidence鈥� in US intelligence agencies, wording that mirrored the White House statement.

But even Secretary Hagel鈥檚 statement reflected a considerable ramping up of the administration鈥檚 public posture on the chemical-weapons issue. Just a day earlier, Hagel had cited what amounted to 鈥渟uspicions鈥� of chemical-weapons use and said the US would not be rushed, by allies or anyone else, in making its own assessment of evidence.

In its letter, the White House said that 鈥済iven the stakes involved,鈥� it is essential that a conclusion be reached based on facts and evidence.

And with the letter talking about 鈥渃orroborated facts鈥� and 鈥渟ome degree of certainty,鈥� there was clear reference to the Iraq war and to the path down which a faulty assessment of Saddam Hussein鈥檚 weapons of mass destruction took the US.