WASHINGTON — Delaying what had loomed as an imminent strike, President Barack Obama abruptly announced Saturday he will seek congressional approval before launching any military action meant to punish Syria for its alleged use of chemical weapons in an attack that killed hundreds.

With Navy ships on standby in the Mediterranean Sea ready to launch their cruise missiles, Obama said he had decided the United States should take military action and that he believes he has “the authority to carry out this military action without specific congressional authorization.”

At the same time, he said, “I know that the country will be stronger if we take this course and our actions will be even more effective.” Congress is scheduled to return from a summer vacation on Sept. 9.

The president didn’t say so, but his strategy carries enormous risks to his and the nation’s credibility, which the administration has argued forcefully is on the line in Syria. Obama long ago said the use of chemical weapons was a “red line” that Syrian President Bashar Assad would not be allowed to cross with impunity.

Only this week, British Prime Minister David Cameron suffered a humiliating defeat when the House of Commons refused to support his call for military action against Syria.

Either way, the developments marked a stunning turn in an episode in which Obama has struggled to gain international support for a strike, while dozens of lawmakers at home urged him to seek their backing.

Halfway around the world, Syrians awoke Saturday to state television broadcasts of tanks, planes and other weapons of war, and troops training, all to a soundtrack of martial music. Assad’s government blames rebels in the Aug. 21 attack, and has threatened retaliation if it is attacked.

Russian President Vladimir Putin, saying he was appealing to a Nobel Peace laureate rather than to a president, urged Obama to reconsider. A group that monitors casualties in the long Syrian civil war challenged the United States to substantiate its claim that 1,429 died in a chemical weapons attack, including more than 400 children.

By accident or design, the new timetable gives time for U.N. inspectors to receive lab results from the samples they took during four days in Damascus, and to compile a final report. After leaving Syria overnight, the inspection team arrived in Rotterdam a few hours before Obama spoke.

The group’s leader was expected to brief Secretary-General Ban Ki-moon on Sunday.

Republicans expressed satisfaction at Obama’s decision, and challenged him to make his case to the public and lawmakers alike that American power should be used to punish Assad.

“We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised,” House Speaker John Boehner of Ohio and other House Republican leaders said in a joint statement.