WASHINGTON (CNN) -- Syria has crossed a 'red line' with its use of chemical weapons, including the nerve agent sarin gas, against rebels, the White House said Thursday.
The acknowledgment is the first time President Barack Obama's administration has definitively said what it has long suspected -- that President Bashar al-Assad's forces have used chemical weapons in the ongoing civil war.
"The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete," Ben Rhodes, the deputy national security adviser for strategic communications, said in a statement released by the White House.
"While the lethality of these attacks make up only a small portion of the catastrophic loss of life in Syria, which now stands at more than 90,000 deaths, the use of chemical weapons violates international norms and crosses clear red lines that have existed within the international community for decades," Rhodes added.
The administration also appeared to indicate that it was stepping up its support of the rebels, who have been calling for the United States and others to provide arms needed to battle al-Assad's forces.
"Put simply, the Assad regime should know that its actions have led us to increase the scope and scale of assistance that we provide to the opposition, including direct support to the (rebel Supreme Military Council). These efforts will increase going forward," Rhodes' statement said.
Rhodes later told reporters on a conference call that the president has made a decision about military support for the rebels, but stopped short of saying the U.S. government would put weapons in the hands of rebels.
He also said no decision has been made by Obama over whether to institute a no-fly zone in Syria, something rebel forces have said is needed to halt al-Assad's aerial bombardment of their strongholds.
The administration also believes that al-Assad's government maintains control of the chemical weapons, and that there is "no reliable, corroborated reporting to indicate that the opposition in Syria has acquired or used chemical weapons."
Rhodes gave no indication of how many times al-Assad's forces used chemical weapons, but a U.S. Senate source briefed on the matter said the administration believes Syria used such weapons on at least eight different occasions.
U.S. Sen. John McCain, who has repeatedly called on the administration to step up its support of the rebels, told CNN's Wolf Blitzer that the rebels need anti-tank and anti-aircraft weapons.
"They need a lot more military assistance," McCain said, adding that the United States and its allies also need to "establish a 'no-fly' zone to create a safe area" within Syria.
"You can't do it with half measures. You can't do it with just supplying weapons," he said.
McCain said the options were not ideal and a response will not be easy. But doing nothing, he said, would be catastrophic.
Earlier this year, the United States said its intelligence analysts had concluded "with varying degrees of confidence" that chemical weapons had been used in the Syrian civil war. But Obama said then that "intelligence assessments alone are not sufficient."
As early as last week, France's foreign minister said sarin gas had been used several times in the Syrian civil war, citing results from test samples in France's possession.
In early May, the head of the U.N. Independent International Commission of Inquiry on Syria said that evidence points to the use of sarin by Syrian rebel forces. But the commission later issued a news release saying it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
In April, the head of the Israeli military's intelligence research said the Syrian government is using chemical weapons against rebel forces.
Barbara Starr and Jessica Yellin reported from Washington, and Chelsea J. Carter reported and wrote from Atlanta. CNN's John Crawley and Chandler Friedman contributed to this report.