The Obama administration has concluded that Syrian President Bashar al-Assad's forces used chemical weapons in that nation's bloody civil war, thus crossing a "red line" that Obama said would trigger a tougher U.S. response.

In addition, the administration announced Thursday that the United States would be for the first time sending direct military aid to the rebel groups fighting Assad for control of the country.

In a statement, Ben Rhodes, a deputy national security adviser, said intelligence assessments had given the administration "high confidence" that Assad had used chemical weapons, which led the U.S. to increase the "scope and scale" of its assistance to the Syrian rebels.

The president has been clear that the use of chemical weapons — or the transfer of chemical weapons to terrorist groups — is a red line for the United States, as there has long been an established norm within the international community against the use of chemical weapons. Our intelligence community now has a high confidence assessment that chemical weapons have been used on a small scale by the Assad regime in Syria. The president has said that the use of chemical weapons would change his calculus, and it has. [White House]

Obama said in April that the U.S. had proof chemical weapons had been used in Syria, but that it did not yet know who used them. That led some to question whether the president was dragging his feet to avoid making a weighty political decision.

Now, Rhodes said the administration had confirmed through "multiple, independent streams of information" that Assad's forces used those weapons, including the nerve agent sarin, multiple times in the past year. Exposure to such weapons resulted in 100 to 150 deaths, with the actual tally likely being higher.

There was "no reliable, corroborated reporting" to indicate the rebels were also using chemical weapons, Rhodes added.

Exactly what shape American military assistance will take is yet to be seen. Rhodes left open the possibility that the U.S. would further broaden its operations in the future, saying there were "other legal, financial, diplomatic, and military responses available."

"We are prepared for all contingencies, and we will make decisions on our own timeline," he added.

Sens. Lindsey Graham (R-S.C.) and John McCain (R-Ariz.), who have advocated a more aggressive American intervention, said in a joint statement that it was "not the time to merely take the next incremental step."

"A decision to provide lethal assistance, especially ammunition and heavy weapons, to opposition forces in Syria is long overdue," they said.

The administration was reportedly considering a plan to arm the rebels back in late April. The administration previously resisted that option over fears that it could not track the weapons adequately and that they would be turned against Americans, though improved relations with rebel factions have reportedly eased those concerns.

On the Senate floor Thursday, McCain — who slipped into Syria weeks ago to meet with rebels — called for Obama to establish a no-fly zone. However, he cautioned that boots on the ground would be "counterproductive."

In a forum with McCain one day earlier, former President Bill Clinton agreed with him that Obama was slow-walking the issue, saying the president should "see down the road" and do more to give the rebels a shot at toppling Assad.

Still, it remains unclear whether the American public would go along with greater U.S. military involvement in Syria. Here's Hayes Brown at Think Progress on that issue:

Recent polls have showed that while a plurality of Americans support the use of force in the face of Assad using chemical weapons, they remain uncertain if not outright opposed towards military intervention in Syria more broadly. [Think Progress]