The White House told reporters on Thursday that it had corroborated evidence of the deployment of chemical weapons during the Syrian civil war. President Barack Obama had previously said proof of the illegal agents would constitute crossing a "red line" and trigger a reaction from the US.

"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," White House natural security adviser Ben Rhodes told reporters.

According to US intelligence, up to 150 people members of the Syrian opposition have died in chemical weapons attacks during the past year.

"The president has said that the use of chemical weapons would change his calculus, and it has," Rhodes added.

Growing evidence from the international community and a change in policy toward Syria among his allies had failed to sway the US president earlier this year. Late last month, the EU allowed part of an arms embargo on Syria to expire so that member states could decide independently whether to ship weapons to the opposition fighters. Obama had maintained his stance, insisting that investigations needed to be substantiated before he would act.

The announcement followed hours after the United Nations appealed for action to end the violence in Syria. At least 93,000 people have died since the civil war began in March 2011, according to figures released by the UN on Friday.

Arms for the opposition

The Obama administration anounced that it had decided to provide military assistance to the opposition.

"We are prepared for all contingencies, and we will make decisions on our own timeline," Rhodes told reporters, declining to detail when or what type of military support would go to the opposition.

Obama planned to consult with US lawmakers and international leaders at the G8 summit in the Northern Ireland next week.

US Republican Senators John McCain and Lindsey Graham urged Obama to act quickly and decisively following the announcement. Their call does not reflect the opinion of all members of Congress, however, who have remained divided over how to bring the Syrian civil war to an end.

Opponents wish to avoid a costly excursion after the huge debts and loss of life in Iraq and Afghanistan. They also fear the consequences for the United States and the Middle East of arming fighters with ties to al-Qaeda militants.

kms/jm (AP, AFP, Reuters, dpa)