WASHINGTON - 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.
"It is such a muddled picture," said Rep. Hal Rogers, R-Ky., chairman of the House Appropriations Committee. "I think probably we should be asking the U.N. to be involved. I think perhaps that's in the making."
Rep. Dutch Ruppersberger of Maryland, the senior Democrat on the House intelligence committee, was among many lawmakers who called for a cautious approach to Syria even as they acknowledged the seriousness of the situation.
"We want to do everything we can to avoid putting boots on the ground," he told reporters. The U.S. should work with other countries to stabilize Syria and ensure its chemical weapons are kept out of the hands of terrorist groups, he said.
"I don't think that we, just as the United States, want to go in to another war," Ruppersberger said.
Obama's vow that Syria's use of chemical weapons would elicit a strong response and the administration's latest caution could raise questions about Obama's definition of a "red line." The U.S. credibility and international authority are on the line in the administration's handling of Syria, and the message it sends to Assad and other nations.
"There's no question that when the United States takes a position that this crosses a line that our failure to respond has implications," said Rep. David Cicilline, D-R.I., a member of the House Foreign Affairs Committee. "I think the president was saying the use of chemical weapons is a game changer. I think most people agree with that. So that if we in fact determine that chemical weapons were used, I think the expectation is that we and the coalition and others take some action."
Rep. John Mica, R-Fla., wondered whether the red line is "turning into a pink line."
In Syria, officials rejected the U.S. intelligence assessment and denied that it had used chemical weapons.
Pressed on the response, State Department spokesman Patrick Ventrell said: "If the regime has nothing to hide they should let the U.N. investigators in immediately so we can get to the bottom of this."
The White House faces a limited choice of military options to help the rebels oust Assad.
Arming the rebels would run into several problems. For one, a military group fighting alongside them has pledged allegiance to al-Qaida. Also, establishing a no-fly zone poses a significant challenge, as Syria possesses an air defense system far more robust than the U.S. and its allies overwhelmed in Libya two years ago.
The next move on Syria was high on the agenda for Obama's meeting Friday with King Abdullah, as the U.S. ally has struggled with the influx of hundreds of thousands of refugees escaping the Syrian violence.
"I think it's important for the administration to look for ways to up the military pressure on Assad," said Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee.
One of the most powerful of the rebel groups in Syria is Jabhat al-Nusra, which recently declared its affiliation with al-Qaida. Last December, the State Department designated the group a terrorist organization, and the administration's opposition to directly arming the Syrian opposition stems from concerns about the weapons ending up in the hands of Islamic extremists.
Arming the rebels, said Sen. Lindsey Graham, R-S.C., is a "lot harder that it was before."
"We've gotten to the point now where the opposition has been affected by the radicals," Graham said in an interview. "Right weapons in right hands is the goal. The second war is coming. I think we can arm the right people with the right weapons. There's a risk there, but the risk of letting this go and chemical weapons falling into radical Islamists' hands is the greatest risk."
Several lawmakers, including Sen. John McCain, R-Ariz., have called for the U.S. to create a narrow, safe zone inside Syria, along its border with Turkey.
Either a safe zone or a no-fly zone would require neutralizing Syria's air defenses. According to a report by the Institute for the Study of War, Syria's largely Soviet-era air defense system includes as many as 300 mobile surface-to-air missile systems and defense systems, and more than 600 static missile launchers and sites.
The U.S. has taken only minimal military steps so far, limiting U.S. assistance to nonlethal aid, including military-style equipment such as body armor and night vision goggles.
The U.S. has deployed about 200 troops to Jordan to assist that country's military, and participated in NATO's placement of Patriot missile batteries in Turkey near the border to protect against an attack from Syria.