President Obama today said the U.S. was conducting a "very vigorous investigation" into the use of chemical weapons in Syria and reiterated that the use of such agents would be a "game changer."

"We cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations," the president told reporters in his first public remarks since the administration revealed Thursday that the Syrian government has likely used chemical weapons against its own people.

(Image Credit: Pablo Martinez Monsivais/AP Photo)

"I meant what I'd said, and I will repeat that it's, obviously, horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed. 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," he said.

The president stressed that more concrete evidence was needed to determine if the Assad regime had crossed his "red line."

"These are preliminary assessments; they're based on our intelligence gathering. 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," he told reporters in the Oval Office where he was meeting with King Abdullah of Jordan.

"We have to act prudently. We have to make these assessments deliberately," Obama said.

The president made clear this was not something that would be solved overnight. "This is not an on or off switch. This is an ongoing challenge that all of us have to be concerned about," he said.

"We're going to be consulting with our partners in the region as well as the international community and the United Nations to make sure that we are investigating this as effectively and as quickly as we can," he said.