WASHINGTON (AP) — President Barack Obama said Friday that any use of chemical weapons by Syria would be a "game changer," but he cautioned that the United States needs more evidence that President Bashar Assad has used the deadly agents against his people.

"We cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations," Obama said in his first comments since the White House disclosed that U.S. intelligence indicates Syria probably has used chemical weapons.

However, the president said more evidence of such use was still needed, including when and how the deadly agents might have been used. He said the U.S., along with the United Nations, would seek to "gather evidence on the ground" in Syria to solidify intelligence assessments.

"This adds increased urgency to what already is a significant security and humanitarian problem in the region," he said from the Oval Office, where he was meeting with Jordan's King Abdullah II.

The White House said on Thursday that the Syrian government probably had used chemical weapons, most likely the agent sarin, in the two-year civil war that has killed more than 70,000 people and displaced hundreds of thousands more.

Obama's cautious response reflects a lack of agreement in Washington over aggressive military intervention. However, lawmakers in both parties also have expressed concern that inaction could embolden not only Assad but such countries as North Korea and Iran as well.

Obama has declared that the Assad government's use of chemical weapons would cross a "red line" for a major military response.

"For the Syrian government to utilize chemical weapons on its people crosses a line that will change my calculus and how the United States approaches these issues," the president said.

Emerging from a closed-door briefing with Secretary of State John Kerry on Capitol Hill, House Republicans and Democrats expressed uncertainty about the appropriate next step as the Obama administration considers limited military options.

No lawmaker pressed for military invasion by the U.S., after more than 10 years of war in Iraq and Afghanistan.