Updated 3:35 PM
While the drumbeat for U.S. action in Syria grew, the White House continued to urge caution as it examines intelligence that the country used chemical weapons on its people.
"These are preliminary assessments," President Obama said before a meeting with King Abdullah II of Jordan Friday. "We have varying degrees of confidence about the actual use, but there are a range of questions around how, when, where these weapons may have been used."
White House Press Secretary Jay Carney told reporters Friday that the U.S. intelligence community is "working to establish credible and corroborated facts...to establish a definitive judgment as to whether the president's red line has been crossed."
"It is absolutely the correct thing to do to take the exceptional work that our intelligence community does and continue to build information," Carney said.
The White House on Thursday confirmed what others in the international intelligence community had already said: Syrian President Bashar al-Assad appears to have used chemical weapons against his own people in the ongoing bloody, two-year civil war.
A White House official said Thursday that the abundance of caution stems from the gravity of the charges. "It's precisely because we take this red line so seriously that we believe there is an obligation to fully investigate any and all evidence of chemical weapon use within Syria," the official told NBC News.
But with U.S. intelligence confirmation, the world--and Congress--is watching to see if President Obama makes good on his promise to take unspecific action on Syrian strongman Assad, the alleged perpetrator of the attacks.
“A red line for us is, we start seeing a whole bunch of chemical weapons moving around or being utilized [in Syria]," Obama said last year. "That would change my calculus.”
What level of U.S. involvement the president would approve is unknown.
The Syrian government has denied the charges. Syrian Minister of Information Omran al-Zoubi said in an interview with Russia Today TV that the Syrian government and armed forces have not and will not use any chemical weapons. He suggested that armed terrorist groups used the chemical weapons.
Hanging over the White House is the shadow of the nearly decade-long Iraq War, when U.S. troops invaded the country on faulty evidence of weapons of mass destruction.
Defense Secretary Chuck Hagel expressed concerns over remaining gaps in the intelligence reports, saying, "We still have some uncertainties about what was used, what kind of chemicals were used, where it was used, who used it." Administration sources confirmed to NBC News that they have not been able to prove the answers to those queries.
While the U.S. works to fill in the intelligence gaps and corroborate with foreign intelligence sources to confirm the chemical attacks, lawmakers warned that the cautious approach demonstrated by the White House could spur an emboldened Assad to call the president's bluff.
"The president of the United States said that if Bashar Assad used chemical weapons, it would be a game-changer, that it would cross a red line. I think it’s pretty obvious that a red line has been crossed,” Sen. John McCain told reporters Thursday. “We have to have operational capability to secure these chemical weapon stocks...We do not want them to fall into the wrong hands, and the wrong hands are a number of participants in the struggle that’s taking place in Syria.”
Republican Sen. Marco Rubio also called on Obama to "explain to Congress and the American people" how he plans on securing Syria's weapons stockpiles, and "what additional measures he is ready to take to follow through on his previous statements."
Senate Intelligence Committee Chair Dianne Feinstein, D-Calif., argued that "red lines have been crossed" in Syria. While she said "action must be taken" in response, she called on an international effort.
“The world must come together to prevent this by unified action which results in the secure containment of Syria’s significant stockpile of chemical weapons,” Feinstein said.