WASHINGTON — President Obama said Friday that he would respond “prudently” and “deliberately” to evidence that Syria had used chemical weapons, tamping down any expectations that he would take swift action after an American intelligence assessment that the Syrian government had used the chemical agent sarin on a small scale in the nation’s civil war.

Mr. Obama’s remarks, before a meeting here with King Abdullah II of Jordan, laid bare the quandary he now faces. The day after the White House, in a letter to Congressional leaders, said that the nation’s intelligence agencies had assessed “with varying degrees of confidence” that the Syrian government had used sarin, the president said he was seeking further proof of culpability for chemical weapons attacks. It is a laborious process that analysts say may never produce a definitive judgment. But Mr. Obama is also trying to preserve his credibility after warning in the past that the use of chemical weapons would be a “game changer” and prompt a forceful American response.

“Knowing that potentially chemical weapons have been used inside of Syria doesn’t tell us when they were used, how they were used,” Mr. Obama told reporters in the Oval Office. “We have to act prudently. We have to make these assessments deliberately.”

“But I meant what I’d said,” the president added. “To use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law. And that is going to be a game changer.”