WASHINGTON – Proceeding cautiously, President Obama insisted Friday that any use of chemical weapons by Syria would change his “calculus” about U.S. military involvement in the two-year-old civil war — but he 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 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.”

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.

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.”