The White House said Thursday that Syrian President Bashar al-Assad has killed somewhere between 100 and 150 dissidents with chemical weapons, crossing what President Barack Obama has referred to as a “red line” that would escalate U.S. involvement in the country’s bloody civil war.

Officials said Obama was considering both political and military options, but it was unclear how quickly new actions would be taken and what they would involve.

Red Line Crossed? White House Acknowledges Syria Used Chemical Weapons, Puts Death Toll at 150

Syrian rebels. (Getty Images)

“We’ve prepared for many contingencies in Syria,” said Ben Rhodes, Obama’s deputy national security adviser. “We are going to make decisions on further actions on our own timeline.”

The White House said the Assad regime had used chemical weapons, including the nerve agent sarin, on a small scale multiple times in the last year. Up to 150 people have been killed in those attacks, the White House said, constituting a small percentage of the 93,000 people killed in Syria over the last two years.

The Obama administration announced in April that it had “varying degrees of confidence” that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.

The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which along with Britain, announced it had determined that Assad’s government had used chemical weapons in the two-year conflict.

Obama has said repeatedly that the use of chemical weapons would cross a “red line” and constitute a “game changer” for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.

The White House said Congress has been notified of the new U.S. chemical weapons determination, as have international allies. Obama will discuss the assessments, along with broader problems in Syria, next week during the G-8 summit in Northern Ireland.

Obama is also expected to press Russian President Vladimir Putin, Assad’s most powerful backers, to drop his political and military support for the Syrian government.

“We believe that Russia and all members of the international community should be concerned about the use of chemical weapons,” Rhodes said.

House Speaker John Boehner’s spokesman Brendan Buck added: “It is long past time to bring the Assad regime’s bloodshed in Syria to an end.”

“As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action,” he said.

Meanwhile, Sens. John McCain (R-Ariz.) and Lindsay Graham (R-S.C.), both who have supported the idea arming the Syrian rebels, released the following statement:

We appreciate the President’s finding that the Assad regime has used chemical weapons on several occasions. We also agree with the President that this fact must affect U.S. policy toward Syria. The President’s red line has been crossed. U.S. credibility is on the line. Now is not the time to merely take the next incremental step. Now is the time for more decisive actions.

The conflict in Syria has taken a dramatic turn for the worse. The United Nations reported today that the death toll in Syria is approaching 93,000. Hezbollah is all in. Iran is all in. Iraqi militant groups are flowing into Syria to fight for Assad. Russia continues to provide military and diplomatic support. Assad is dramatically increasing his use of airpower against civilians and opposition forces in Syria. A report today stated that Assad’s forces conducted at least 5,000 air-to-ground attacks last month alone.

Red Line Crossed? White House Acknowledges Syria Used Chemical Weapons, Puts Death Toll at 150

Getty Images.

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. But providing arms alone is not sufficient. That alone is not enough to change the military balance of power on the ground against Assad. The President must rally an international coalition to take military actions to degrade Assad’s ability to use airpower and ballistic missiles and to move and resupply his forces around the battlefield by air. This can be done, as we have said many times, using stand-off weapons such as cruise missiles.

We cannot afford to delay any longer. 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.”

You can read the full text of the U.S. statement on Assad’s use of chemical weapons here.

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The Associated Press contributed to this article. This post has been updated.