WASHINGTON (AP) — President Barack Obama is facing a new quandary from a pair of assertive allies, France and Britain, that suggest his stated “red line” for more forceful US action in the Syrian civil war has been crossed with solid evidence of chemical weapons use by the Assad regime.

Mindful of America’s own checkered intelligence record, US officials insist they still lack incontrovertible proof that Syrian President Bashar Assad ordered the use of chemical weapons. Even after France’s declaration that it has “no doubt” about Assad’s hand in at least one chemical attack, the Obama administration isn’t talking about intervening in Syria’s 27-month conflict that has drawn in Hezbollah and al-Qaida-linked militants on opposing sides.

The administration fears plunging the United States into an Iraq-like calamity based on misleading or incomplete evidence — or getting involved militarily at all.

“Make no mistake whatsoever, the president’s red line is real, the president said it would be a game changer,” Secretary of State John Kerry told reporters in Guatemala on Wednesday, adding that France was sending its chemical weapons evidence to the US for review. “The president has a whole set of options on the table and all of them are alive.”

At the White House, press secretary Jay Carney cautioned that the US still needed to see evidence that “makes a concrete case for the assertion that chemical weapons have been used, that can demonstrate when and by whom they were used and the consequences of that use.”

For the United States, the stakes are high. Since Obama declared last summer that Assad’s use of chemical weapons or transfer to extremist groups such as Hezbollah would constitute a “red line” that would have “extraordinary consequences,” the United States has left open the possibility of all actions short of American military boots on the ground. Even as more than 70,000 people have been killed and Assad’s forces have made significant gains against the rebels, the administration has played down threats of any armed action as long as Syria’s vast chemical weapons stockpiles remained stowed away.

Swedish chemical weapons expert Ake Sellstrom, who heads the UN investigation team appointed by Secretary-General Ban Ki-moon, also has questions about the evidence he received Tuesday from the French government.

“Sellström cautions that the validity of the information is not ensured in the absence of convincing evidence of the chain-of-custody of the data collected,” UN spokesman Martin Nesirky said.

Sellstrom and the secretary-general again urged Syria to allow the UN team into the country, stressing that “on-site activities are essential if the United Nations is to be able to establish the facts,” Nesirky said.

Firming up evidence of such weapons use is thus paramount, especially after the US spent more than $800 billion and lost almost 5,000 soldiers in an Iraq war justified on exaggerated claims about Saddam Hussein’s nuclear ambitions and weapons of mass destruction programs. It is also a question of principle for Obama, who became commander-in-chief vowing opposition to “dumb wars” but showed himself willing to join with U.S. allies and partners in a limited mission in 2011 to overthrow Libyan dictator Moammar Gaddafi.

“We have a history, of course, on issues like this in the United States, and we all remember what happened around Iraq,” State Department spokeswoman Jen Psaki said. “We want to make sure what the facts are certain before we make a conclusion.”

Several US officials say the administration is being deliberately ambiguous about what it knows of chemical weapons use in Syria. They cite a variety of reasons: the weak appetite among Americans for another war in the Arab world, a lack of good military options against a Syrian army with far stronger air defenses than Gaddafi’s, and lingering hopes a peace process might take hold between the Assad regime and the Syrian opposition.

Developments beyond Washington’s control, however, are chipping away at the US position. After the regime and rebels traded accusations of chemical weapons use in Aleppo and Damascus in March, the administration sought to avoid any rash judgment. It took the British and French going public with their initial assessments of regime attacks to prompt a U.S. acknowledgement of similar intelligence.

It’s unclear if a similar process might occur now. On Tuesday, French Foreign Minister Laurent Fabius said he had confirmation of multiple uses of sarin and at least once definitively by the regime, while the British cited positive tests for the same nerve agent. Both findings, based on samples taken from Syria, came hours after a UN team said it had “reasonable grounds” to suspect small-scale use of toxic chemicals in at least four attacks in March and April.

“We had this information. We had to publish it,” Fabius told French television on Wednesday.

British Foreign Secretary William Hague said on BBC radio that the evidence is “very strong.”

Both called for a UN investigating team to be allowed into Syria to take further samples. Assad is unlikely to grant unfettered access.

U.S. officials say they, too, have evidence of chemical weapons attacks, but they note reservations about the “chain of custody” and questions about who used the agents, how much was used and under what circumstances. No one in the administration believes the opposition either has chemical stockpiles or the know-how to deploy them, but they say more work needs to be done before any case for clear violations can be publicly and compelling defended.

The biggest challenge concerns what to do if and when the U.S. states unequivocally that chemical weapons were used.

At NATO headquarters on Wednesday, Defense Secretary Chuck Hagel said he’d not seen the new French evidence and that in the talks “we didn’t get into any additional war plans regarding Syria.”

Obama called for Assad to step down almost two years ago to allow for a more democratic Syria to emerge. Since then, the US has given the opposition more than $120 million in nonlethal aid but has taken no significant action to ensure Assad’s departure. Others such as Kerry have spoken of increased support to the rebels to convince Assad he can’t win the war. Yet as American officials debate the merits of sending weapons, the Assad regime has succeeded in regaining more territory through a military counteroffensive backed by allied Iranian and Hezbollah forces.

If the US announces that chemical weapons were used, and then doesn’t offer any immediate consequences, it could suffer a damaging loss of credibility in a part of the world still rife with unstable states and where Obama has presented another “red line” to Iran, concerning its disputed nuclear program.

A US national security shake-up announced Wednesday could drive Obama into a more forward-leaning posture. UN Ambassador Susan Rice, who supported military force in Libya two years ago, is replacing Tom Donilon as national security adviser. Samantha Power, a human rights advocate and former White House adviser who has championed intervention to prevent genocide and other atrocities, is coming in for Rice at the global body.

___

Associated Press writers Lolita C. Baldor in Brussels, and Angela Charlton and Greg Keller in Paris contributed to this report.

Copyright 2013 The Associated Press.