WASHINGTON » President Barack Obama today declared unequivocally that the United States has "concluded" that the Syrian government carried out a deadly chemical weapons attack on civilians. But new hurdles emerged that appeared to slow the formation of an international coalition that could use military force to punish Syria.

Obama did not present any direct evidence to back up his assertion that the Syrian government bears responsibility for the attack. While he said he is still evaluating possible military retaliation, the president vowed that any American response would send a "strong signal" to Syrian President Bashar Assad.

"We have concluded that the Syrian government in fact carried these out," Obama said during an interview with PBS’ NewsHour. "And if that’s so, then there need to be international consequences."

Earlier today, the five permanent members of the United Nations Security Council failed to reach an agreement on a draft resolution from the British seeking authorization for the use of force. Russia, as expected, objected to international intervention.

Obama administration officials said they would take action against the Syrian government even without the backing of allies or the United Nations because diplomatic paralysis must not prevent a response to the alleged chemical weapons attack outside the Syrian capital last week.

Despite the administration’s assertions that it would press forward without the U.N., momentum for international military action appeared to slow.

British Prime Minister David Cameron promised British lawmakers he would not go to war until a U.N. chemical weapons team on the ground in Syria has a chance to report its findings, pushing the U.K.’s involvement in any potential strike until next week at the earliest. Cameron called an emergency meeting of Parliament on Thursday to vote on whether to endorse international action against Syria.

Even so, British Foreign Secretary William Hague suggested that U.S. military action need not be constrained by Britain. "The United States are able to make their own decisions," he told reporters today, just after speaking with Secretary of State John Kerry.

U.S. officials were in search of additional intelligence to bolster the White House’s case for a strike against Assad’s military infrastructure.

American intelligence intercepted lower-level Syrian military commanders’ communications discussing a chemical attack, but the communications don’t specifically link the attack to an official senior enough to tie the killings to Assad himself, according to one U.S. intelligence official and two other U.S. officials. They spoke on condition of anonymity because they were not authorized to discuss the intelligence publicly.

The White House ideally wants intelligence that links the attack directly to Assad or someone in his inner circle, to rule out the possibility that a rogue element of the military decided to use chemical weapons without Assad’s authorization.

That quest for added intelligence has delayed the release of the report by the Office of the Director for National Intelligence laying out evidence against Assad. The report was promised earlier this week by administration officials.

The CIA and the Pentagon have been working to gather more human intelligence tying Assad to the attack, relying on the intelligence services of Jordan, Saudi Arabia and Israel, the officials said. The administration was planning a teleconference briefing Thursday on Syria for leaders of the House and Senate and national security committees in both parties, U.S. officials and congressional aides said.

Both the CIA and the Defense Intelligence Agency have their own human sources — the rebel commanders and others who cross the border to brief CIA and defense intelligence officers at training camps in Jordan and Turkey. But their operation is much smaller than some of the other intelligence services, and it takes longer for their contacts to make their way overland.

The CIA, the Pentagon, and the Office of the Director of National Intelligence all declined to comment on the intelligence picture, and the White House did not respond to requests for comment.

Obama said he was not seeking a lengthy, open-ended conflict in Syria, indicating that any U.S. response would be limited in scope. But he argued that Syria’s use of chemical weapons not only violated international norms, but threatened "America’s core self-interest."

"We do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable," he said.

Laying out a legal justification for a U.S. response, Obama said Syria was violating the Geneva Protocols, an agreement signed in 1925 in the wake of World War I to ban the use of chemical gases. The White House has also cited the Chemical Weapons Convention, a 1992 agreement that builds on the Geneva Protocols by prohibiting the development and stockpiling of chemical weapons.

Syria is a party to the original Geneva accord, but not the latter chemical weapons agreement.

Syria, which sits on one of the world’s largest stockpiles of chemical weapons, has denied the charges. Moreover, Syria’s U.N. ambassador, Bashar Ja’afari, is demanding that United Nations experts investigate three alleged chemical weapons attacks against Syrian soldiers. He said the attacks occurred on Aug. 22, 24 and 25 in three suburbs of the Syrian capital and dozens of soldiers are being treated for inhaling nerve gas.

Certain members of Congress are expected to get a classified U.S. intelligence report laying out the case against Assad. An unclassified version is to be made public. Officials say it won’t have any detail that would jeopardize sources and methods.

Some lawmakers have argued that Congress must authorize any military action unless there has been an attack on the U.S. or the existence of an eminent threat to the U.S. Both Democrats and Republicans today pressed the White House to provide a clear explanation of how military action would secure U.S. objectives.

Specifically, in a letter to Obama, House Speaker John Boehner asked him to make his case to Congress and the public about how military action would "secure American national security interests, preserve America’s credibility, deter the future use of chemical weapons, and, critically, be a part of our broader policy and strategy."

Boehner said it was "essential you address on what basis any use of force would be legally justified."

AP White House Correspondent Julie Pace and AP writers Raphael Satter and Greg Katz in London contributed to this report.