U.S. President Barack Obama repeated on Friday his past assertions that the use of chemical weapons in Syria would cross a line and produce an American response.

At the same time, he indicated that he was not yet 100% certain that the findings by U.S. intelligence agencies are an indication that such weapons had indeed been used.

“Knowing that there’s chemical weapons in Syria doesn’t tell us when they were used or how they were used,” Obama told reporters before a meeting with King Abdullah II of Jordan which focused on Syria.

“We ourselves will be putting a lot of resources on this.”

Asked about a timeline, Obama added, “This is going to be a long-term proposition, not something we could solve easily one night.”

“To use weapons of mass destruction on civilian populations crosses another line in terms of international norms and laws,” he said. “That’s going to be a game changer.”

“For the Syrian government to use chemical weapons on its people will change my calculus,” he added. “This is not an on and off switch. It’s an ongoing challenge that all of us have to work with.”

Earlier, White House press secretary Jay Carney said Obama is awaiting a “definitive judgment” on whether the Syrian regime used chemical weapons against rebel fighters before taking action.

“We’re working to establish credible and corroborated facts,” White House spokesman Jay Carney told reporters.

“The president wants the facts,” he added, saying there was no timeline for further action by the United States.

British Prime Minister David Cameron said on Friday that growing evidence of the use of chemical weapons by the Syrian regime was “extremely serious”.

Cameron agreed with President Barack Obama that such use would represent a “red line” for the international community, but said the response would likely be political rather than military.

Syria’s opposition called on the international community on Thursday to act “urgently and decisively” on Obama’s public warning that Syria’s use of chemical weapons would cross a “red line.”

A statement by the Syrian National Coalition called on the world community to prove to the Syrian regime that the statements about crossing a red line were not just “empty words.”