WASHINGTON— The Obama administration announced on Thursday that it has determined that the Syrian government has deployed chemical weapons against opposition groups, crossing what President Obama had called a "red line" and prompting him to provide direct support the Syrian groups for the first time.

While the White House stopped short of saying it would arm Syrian rebels, Deputy National Security Adviser Ben Rhodes said that the president has decided to step up "military support" to the main opposition group , the Supreme Military Council, to bolster their effectiveness.

The announcement comes ahead of next week's Group of Eight Summit in Northern Ireland, where the conflict in Syria is expected to be a focal point of conversation. Russia, one of the G-8 member countries, has continued to back the Bashar Assad regime, despite pleas from the United States and international community to cease.

The White House has also expressed concerns that the situation is getting more dire in Syria as Hezbollah and Iran have stepped up their involvement in the conflict in support of Assad.

"There is an urgency to the situation," Rhodes said. "There has been an urgency to the situation for two years. It's particularly urgent right now in terms of the situation on the ground, in some respect, because we have seen Hezbollah and Iran increase their own involvement in the conflict."

The White House notified lawmakers on Thursday that they had positively determined chemical weapons were used before announcing the findings to the press.

Obama acknowledged in April that chemical weapons likely had been deployed, but they needed further confirmation before taking action. Obama had called the potential use of chemical weapons by Syrian President Bashar Assad a "red line" that would spur further action by the U.S.

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

The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date, a small fraction of the more than 90,000 that have been killed in the two-year-old civil war. The U.S. assessment is based on laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin.

In a speech on the Senate floor, Sen. John McCain suggested that Obama has already decided to arm the rebels. But Rhodes said that the White House will offer "direct support" to Syrian rebels to increase their effectiveness, but said he could not "inventory the types of support" at this point.

Saudi Arabia, Qatar and other Gulf Arab states are already providing arms and ammunition to rebel groups. What is lacking is cohesion and organization. But Assad's forces have made recent headway against the rebels, driving them from a strategic city near the border of Lebanon.

"You need to provide the right kind of arms," said Brian Rogers, a spokesman for McCain. "What they really need is anti-armor and anti-aircraft weapons."

Rhodes said that the president has not made any decisions to implement a no-fly zone--as he did in Libya in as part of an international effort to oust Muammar Gaddafi. But Rhodes underscored that White House officials believe the most effective action they can take to improve the situation on the ground is to strengthen the opposition.

"A no-fly zone...would carry with it great and open-ended costs for the United States and the international community," Rhodes said. "It would be far more complex to undertake that effort in Syria than it was in Libya. Furthermore, there's not a clear guarantee that it would dramatically improve the situation on the ground."

Several lawmakers applauded the president for acknowledging the chemical weapon use, but called on him to take more significant action to topple Assad.

"I thank the president for acknowledging that Syrians are using chemical weapons," McCain said. "Just to provide additional weapons to the Syrian National Army is not enough. We have got to change the equation on the battleground."

"It is in our national security interest to see that the war in Syria ends and Assad is displaced," said Sen. Lindsey Graham, R-S.C., "It can be done, and it should be done, and it is in our interest to do it."

Earlier on Thursday, Politico reported that former President Bill Clinton agreed with McCain that Obama should be more forceful in his support of the Syrian rebels.

"Some people say, 'Okay, see what a big mess it is? Stay out!' I think that's a big mistake," Clinton said during a Tuesday event on behalf of the McCain Institute for International Leadership in New York City.

"I agree with you about this," Clinton told McCain. "Sometimes it's just best to get caught trying, as long as you don't over-commit -- like, as long as you don't make an improvident commitment."

Contributing: Susan Davis and Catalina Camia