WASHINGTON (AP) -- Proceeding cautiously, President Barack Obama insisted on Friday that any use of chemical weapons by Syria would change his "calculus" about U.S. military involvement in the 2-year-old civil war -- but said too little was known about a pair of likely sarin attacks to order aggressive action now.
The president's public response to the latest intelligence reflected the lack of agreement in Washington over whether to use America's military to intervene in the civil war, -- and if so, how. But lawmakers in both parties expressed concern that inaction could embolden Syrian President Bashar Assad and perhaps other countries including North Korea and Iran.
U.S. officials declared on Thursday that the Syrian government probably had used chemical weapons twice in March, newly provocative acts in the civil war that has killed more than 70,000 people and displaced hundreds of thousands more. The U.S. assessment followed similar conclusions from Britain, France, Israel and Qatar -- key allies eager for a more aggressive response to Syrian conflict.
Obama, in his first comments about the new intelligence disclosure, said Friday, "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." He has issued similar warnings for months, saying the use of chemical weapons or transfer of the stockpiles to terrorists would cross a "red line" and carry "enormous consequences."
Seeking to show resolve, Obama added Friday that "I've meant what I said."
The president is facing political pressure from a familiar contingent of senators, led by Arizona Republican John McCain, favoring a quick and strong U.S. response. But even those lawmakers appear opposed to an American military invasion and are instead supporting creation of a protective "no-fly zone" or another narrow, safe zone inside Syria, along its border with Turkey.
Some lawmakers voiced concern that if Obama doesn't make good on his promise to respond aggressively if it's shown that Assad used chemical weapons, his inaction could send a damaging message to the world.
"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, a Democratic member of the House Foreign Affairs Committee. "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 was "turning into a pink line."
White House officials insisted Obama's caution was not an indication that the line was shifting. Officials said firm evidence of a chemical weapons attack would trigger a U.S. response -- unspecified -- and would not be contingent on the size and scope of the use.
Obama met at the White House with Jordan's King Abdullah II, whose nation is suffering amid an influx of refugees spilling over its border with Syria. The president promised to vigorously pursue more information about chemical weapons attacks, including exactly who might be responsible and how they might have been carried out.
But the president set no deadline for answers.
"The president wants the facts," spokesman Jay Carney said. "And I'm not going to set a timeline because the facts need to be what drives this investigation, not a deadline."
Syrian officials denied Friday that their government forces had used chemical weapons against rebels.
Hanging over the Obama administration's approach to the new intelligence reports are hard lessons learned from the Iraq war, when faulty intelligence drew the U.S. into a lengthy and expensive conflict. Obama, as a candidate for U.S. Senate, opposed the Iraq war and made ending the conflict a priority in his first term.
Lawmakers on Capitol Hill appeared to be drawing on similar lessons from more than a decade ago. Many who sounded the alarm about Saddam Hussein and the possibility of weapons of mass destruction -- and strongly stood with President George W. Bush's decision to invade Iraq -- were far more muted on Friday.
Following a closed-door briefing by Secretary of State John Kerry, they stressed the importance of building international support for any military move against Syria rather than unilateral U.S. action. The sectarian strife in Iraq and the lawlessness in Libya after the killing of longtime leader Moammar Gadhafi in 2011 stand as sober reminders of what can happen.
"We want to do everything we can to avoid putting boots on the ground," said Rep. Dutch Ruppersberger of Maryland, the senior Democrat on the House intelligence committee. "I don't think that we, just as the United States, want to go in to another war."
Polling shows war-weary Americans are broadly opposed to the notion of the U.S. military intervening in Syria. Just one in five said the U.S. has a responsibility to do something about the fighting in Syria, according to a CBS News poll conducted in late March.
But faced with more specific scenarios, Americans appear more willing to back U.S. involvement. In an ABC News/Washington Post poll late last year, 63 percent said they would support military intervention if the Syrian government used chemical weapons against its people.
Roughly the same number said they would support using American military aircraft to create a no-fly zone if no ground troops were involved.
The White House faces a limited choice of military options to help the rebels oust Assad.
Arming the rebels would run into the reality that a military group fighting alongside them has pledged allegiance to al-Qaida. 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.
Thus far, the Obama administration has limited its assistance to the Syrian rebels to nonlethal aid, including military-style equipment such as body armor and night vision goggles. The U.S. has also deployed about 200 troops to Jordan to assist that country's military, and has participated in NATO's placement of Patriot missile batteries in Turkey near the border to protect against an attack from Syria.