WASHINGTON -- Evidence is clear that chemical weapons were used against rebel forces in Syria, the White House acknowledged in a call with reporters on Thursday.

As a result, US President Barack Obama has decided to directly arm the rebels "to strengthen their effectiveness," the White House said. The new aid will be "military" in nature, but was not detailed.

Congress and allies were notified by the White House before the announcement.

Deputy National Security Advisor For Strategic Communications Ben Rhodes said that the new assessment has indeed "changed the calculus" of Obama.

"This is an issue that we've been following very closely," Rhodes said. "The Assad regime has used chemical weapons-- sarin-- on a small scale multiple times over the past year."

Some 100 to 150 people have died, though that data on casualties is incomplete, the White House says.

The US has identified attacks as recently as May 23 in Damascus.

The administration says a decades-old "red line" has been crossed, and that the "potential response" of the United States would be mulled over the in the coming days.

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"We've prepared for many contingencies," Rhodes said, "consistent with our own national interests."

Sources within the administration point to the president's growing interest in a limited no-fly zone based out of Jordan that would protect part of the country for rebel training and refugee assistance.

"There has been an urgency to the situation for two years," Rhodes said, but it's "particularly urgent right now" with an increase in Iranian and Hezbollah involvement in the fight.

Russia was also briefed on the new US intelligence.

"In the absence of a political settlement," says Rhodes, considering the current state of outside involvement, "you're going to have conflict in that country until someone prevails in that conflict."

The announcement comes weeks after French and British officials disclosed  similar findings, and as President Obama continued to take meetings throughout the week in the Oval Office on how to turn the tide of the war. Assad has made dramatic gains against opposition forces in recent weeks with the help of thousands of Hezbollah fighters, and is preparing a significant offensive against the rebel stronghold of Aleppo.

In response to the announcement, senators Lindsay Graham and John McCain released a joint statement calling for immediate action.

“We cannot afford to delay any longer," the statement reads. "Assad is on the offensive with every weapon in his arsenal and with the complete support of his foreign allies. We must take more decisive actions now to turn the tide of the conflict in Syria.”

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