WASHINGTON » Delaying what had loomed as an imminent strike, President Barack Obama abruptly announced 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 today 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.

"In consultation with the president, we expect the House to consider a measure the week of September 9th. This provides the president time to make his case to Congress and the American people."

It appeared that effort at persuasion was already well underway.

The administration arranged a series of weekend briefings for lawmakers, both classified and unclassified, and Obama challenged lawmakers to consider "what message will we send to a dictator" if he is allowed to kill hundreds of children with chemical weapons without suffering any retaliation.

While lawmakers are scheduled to return to work Sept. 9, officials said it was possible the Senate might come back to session before then.

Obama said Friday he was considering "limited and narrow" steps to punish Assad, adding that U.S. national security interests were at stake. He pledged no U.S. combat troops on the ground in Syria, where a civil war has claimed more than 100,000 civilian lives.

With Obama struggling to gain international backing for a strike, Putin urged him to reconsider his plans. "We have to remember what has happened in the last decades, how many times the United States has been the initiator of armed conflict in different regions of the world, said Putin, a strong Assad ally. "Did this resolve even one problem?"

Even the administration’s casualty estimate was grist for controversy.

The Syrian Observatory for Human Rights, an organization that monitors casualties in the country, said it has confirmed 502 deaths, nearly 1,000 fewer than the American intelligence assessment claimed.

Rami Abdel-Rahman, the head of the organization, said he was not contacted by U.S. officials about his efforts to collect information about the death toll in the Aug. 21 attacks.

"America works only with one part of the opposition that is deep in propaganda," he said, and urged the Obama administration to release the information its estimate is based on.

Obama was buffeted, as well, by some lawmakers challenging his authority to strike Syria without congressional approval, and also by others who urged him to intervene more forcefully than he has signaled he will.

In the hours before Obama’s Rose garden announcement, he was joined at the White House by top advisers.

Vice President Joseph Biden, who had planned a holiday weekend at home in Delaware, was among them. So, too, were Defense Secretary Chuck Hagel, Secretary of State John Kerry and other top administration officials.

In the famously flammable Middle East, Israel readied for the possible outbreak of hostilities. The Israeli military disclosed it has deployed an "Iron Dome" missile defense battery in the Tel Aviv area to protect civilians from any possible missile attack from next-door Syria or any of its allies.

Missile defenses were deployed in the northern part of the country several days ago, and large crowds have been gathering at gas mask-distribution centers to pick up protection kits.

Associated Press writers Julie Pace, Josh Lederman, Matthew Lee and Kimberly Dozier in Washington; Zeina Karam, Yasmine Saker and Karin Laub in Beirut; and Geir Mouslon in Berlin contributed to this report.

Online:

U.S. government assessment: http://apne.ws/14etnyn