In a sharp escalation of the U.S. role in Syria's bloody civil war, the White House announced late Thursday that it will provide military aid to rebels fighting to topple President Bashar al-Assad after confirming that his government used chemical weapons against the opposition.

Deputy National Security Advisor for Strategic Communications Ben Rhodes told reporters on a conference call that President Barack Obama had heard pleas from Syria's rebel Supreme Military Council (SMC) for more help. "Our aim is to be responsive," Rhodes said, underlining that the new assistance would have "direct military purposes."

Rhodes brushed aside repeated questions about whether this meant Washington would now start providing weapons to the rebels, insisting he could not give an "inventory" of the aid. But while he never explicitly confirmed that Obama had decided to to arm the opposition, he left little doubt about Washington's new course of action.

"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the SMC. That includes military support. I cannot detail for you all of the types of that support for a variety of reasons," Rhodes said. The assistance is "aimed at strengthening the effectiveness of the SMC on the ground."

Obama reached the decision after America's intelligence community concluded 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," Rhodes said. Those attacks killed at least 100-150 people, he added. Rhodes said Assad's forces used chemical weapons on March 19, April 13, May 14 and May 23.

The confirmation—and a new United Nations study that raised the death toll from Syria’s bloody civil war to nearly 93,000—ramped up pressure on Obama to escalate American involvement in the conflict. The president has been weighing whether to arm the opposition, help create safe areas for refugees, or impose “no-fly zones” inside Syria enforced by American-led forces. Obama last year called the confirmed use of chemical weapons a "red line" that would make him reconsider whether to arm the rebels, but he later hedged that statement.

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Republican Sen. John McCain, who for months had publicly pressed Obama to step up U.S. involvement, preempted the White House announcement in remarks, announcing on the Senate floor that U.S. intelligence agencies had confirmed the use of chemical weapons and thanking the president for opting to send weapons to the rebels.

“In just a couple of minutes, the president of the United States will be announcing that it is now conclusive that Bashar Assad and the Syrian butchers have used chemical weapons,” McCain said.

“The president also will announce that we will be assisting the Syrian rebels in Syria by other assistance” but the president “had better understand that just supplying weapons is not going to change the equation on the ground of the balance of power,” the senator added. "These people of the Free Syrian Amry need weapons and heavy weapons to counter tanks and aircraft, they need a no-fly zone."

“Just providing arms is not enough,” McCain said.

(Later, in a joint written statement with Republican Sen. Lindsey Graham of South Carolina, McCain seemed to indicate a decision had not yet been made: “A decision to provide lethal assistance, especially ammunition and heavy weapons, to opposition forces in Syria is long overdue, and we hope the President will take this urgently needed step.")

Separately, The Wall Street Journal reported that a military proposal for getting weapons to the rebels also calls for a "no-fly zone" inside Syria to protect civilians fleeing the fighting and rebels who might train there.

The Journal, citing anonymous officials, said the U.S. military was looking at a “no-fly zone” that would stretched some 25 miles into Syrian territory.

Rhodes emphasized that "we have not made any decision to pursue a military operation such as a no-fly zone.”

Such an effort "would carry with it great and open-ended costs for the United States and the international community," Rhodes said. And it would be "far more complex to undertake that effort in Syria than it was in Libya."

He added, "Furthermore, there's not even a clear guarantee that it would dramatically improve the situation on the ground."

Rhodes said Obama would consult with Congress and American allies on next steps—notably at next week’s summit of the Group of Eight rich countries plus Russia in Northern Ireland. The White House has not given up on a negotiated solution.

A spokesman for Republican House Speaker John Boehner, Brendan Buck, emphasized the need for the administration to keep lawmakers in the loop.

“It is long past time to bring the Assad regime’s bloodshed in Syria to an end,” Buck said. “As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action.”