The Senate and House intelligence committees were briefed by administration officials on the threat in Syria as recently as last week, and several sources familiar with the briefing say lawmakers were told that three people died from sarin poisoning.

These sources could not speak with attribution, because intelligence briefings are confidential and lawmakers and aides are sworn to secrecy.

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The White House publicly acknowledged Thursday that Syria used chemical weapons, leading several senators called on the Obama administration to step up its response.

“It’s pretty obvious that red line has been crossed,” Sen. John McCain (R-Ariz.) told reporters.

“The situation in Syria is unacceptable,” he added. “The president of the United States said that this would be a red line if they used chemical weapons. The president of the United States has now told us they used chemical weapons.”

McCain, who has been advocating more intervention in Syria for years, called for arming the opposition, a step that the White House has resisted thus far, establishing a safe zone and taking proactive steps to ensure that the chemical weapons don’t fall into the wrong hands.

“It does not mean boots on the ground,” McCain said.

Senators received a briefing Thursday morning from several officials, including Secretary of State John Kerry. House lawmakers will receive the same briefing at 9:30 a.m. Friday.

Sen. Dianne Feinstein (D-Calif.), who chairs the Senate Intelligence Committee, said called on the national community to respond to the crossing of “red lines.”

“I am very concerned that with this public acknowledgement, President [Bashar] Assad may calculate he has nothing more to lose and the likelihood he will further escalate this conflict therefore increases,” Feinstein said. “It is also important that the world understands the use of weapons of mass destruction, such as sarin, will not be countenanced, and clearly Assad must go.”

Senate Majority Whip Dick Durbin (D-Ill.) stopped short of saying what steps should be taken, but agreed a line has been crossed.

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“From what I’ve heard our intelligence indicated with some degree of certainty that it has been crossed,” Durbin said. “That’s up to the commander in chief, but something has to be done.”

Sen. Lindsey Graham (R-S.C.) joined McCain in calling for a larger response, saying that it’s important to ensure that potential chemical weapons are secured after the Assad regime falls.

“The day that Assad falls there will be as surely as I’m standing here a conflict between the majority of Syrians who want to move forward and live in peace and an element of radical jihadists and that conflict needs to be planned for and brought to an end,” Graham said. “The sooner Assad leaves, the better for the world.”

Several senators exiting the briefing without knowing that the use of chemical weapons had been publicly disclosed by Defense Secretary Chuck Hagel, refused to comment citing the classified nature of the briefing.

Sen. Carl Levin (D-Mich.), who also signed the letter McCain and Graham sent the White House asking about the use of chemical weapons, opted not to comment on the contents or respond to inquires about whether the U.S. should take more action.

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Sen. Bob Menendez (D-N.J.), chairman of the Foreign Relations Committee, declined to comment on Syria.

Sen. Bob Corker (R-Tenn.), the ranking member of the Foreign Relations Committee, said before action involving troops is taken, the president should consult with the Senate.

“This assessment is deeply troubling, and if correct, means that President Obama’s red line has certainly been crossed,” Corker said in a statement. “While more work needs to be done to fully verify this assessment — like making sure we understand the chain of custody of the evidence — it is becoming increasingly clear that we must step up our efforts.”

The key line in the letter, which McCain quoted from while reading to reporters, acknowledged the use of sarin.

“Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin,” the letter reads.