Members of the Syrian community in Allentown, Pa., rally Wednesday against the United States' involvement in Syria. President Barack Obama said the U.S. has “concluded” that the Syrian government carried out a deadly chemical weapons attack on civilians.

WASHINGTON — President Barack Obama is vowing that American retaliation for Syrian chemical weapons use would send a “strong signal,” as U.S. intelligence officials readied briefings for Congress on evidence aimed at linking last week’s attack to President Bashar Assad’s government.

But even as the U.S. moves closer to possible military action, new hurdles appear to be slowing the formation of an international coalition to undertake military action. And questions remain about the strength of the case against Assad.

Russia blocked British efforts to seek a force resolution at the United Nations. British Prime Minister David Cameron said his country would hold off on joining any military efforts until a U.N. chemical weapons inspection team releases its findings. U.N. Secretary-General Ban Ki-moon said the team is expected to complete its inspection Friday and report to him Saturday; they will share their conclusions with members of the Security Council, Ban said.

“If any action would be taken against Syria it would be an international collaboration,” Defense Secretary Chuck Hagel reiterated today.

Still, the Obama administration vowed to take action even without the backing of allies or the U.N. The president said that while he had not settled on a response to last week’s purported chemical weapons attack near Damascus, the U.S. has concluded that Assad’s regime perpetrated the attack, which killed at least 100 Syrians.

“And if that’s so,” Obama said during an interview with “NewsHour” on PBS, “then there need to be international consequences.”

Obama did not present specific evidence to back up his assertion that the Assad regime is responsible for the Aug. 21 attack.

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

The administration was planning an intelligence teleconference briefing today on Syria for leaders of the House and Senate and the national security committees in Congress, U.S. officials and congressional aides said.

Britain added a hurdle to deliberations about a military strike on Wednesday when it went to the U.N. Security Council with a draft resolution that would authorize the use of military force against Syria. The British resolution would authorize “all necessary measures under Chapter 7 of the U.N. Charter to protect civilians from chemical weapons.” Chapter 7 allows the use of international armed force to back up U.N. decisions.

As expected, the five permanent members of the Security Council failed to reach an agreement as Russia reiterated its objections to international intervention in the Syrian crisis. Russia, along with China, has blocked past attempts to sanction the Assad government.

Obama said he was not seeking a lengthy, open-ended conflict in Syria, indicating any U.S. response would be limited in scope. But he argued that Syria’s use of chemical weapons also 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.