Nor was it clear what options would be open to the president if he fails to win the backing of the House and Senate for the military measures he has threatened.
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 UN 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 generally 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."
There was one dissenting view, from Rep. Peter King., R-N.Y. "President Obama is abdicating his responsibility as commander in chief and undermining the authority of future presidents," he said. "The president doesn't need 535 Members of Congress to enforce his own red line."
Senior administration officials said Obama told aides on Friday night that he had changed his mind about ordering a strike against Syria without seeking congressional approval first, making a final decision after a long discussion with his chief of staff Denis McDonough.
It was unclear what pressure Republican or Democratic lawmakers had brought on Obama.
For now, it appeared that the administration's effort at persuasion was already well underway.
The administration plunged into 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.
At the same time, a senior State Department official said Secretary of State John Kerry spoke with Syrian Opposition Coalition President Ahmed Assi al-Jarba to underscore Obama's commitment to holding the Assad government accountable for the Aug. 21 attack.
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 US national security interests were at stake. He pledged no US combat troops on the ground in Syria, where a civil war has claimed more than 100,000 civilian lives.
In Syria, some rebels expressed unhappiness with the president, one rebel commander said he did not consider Obama's decision to be a retreat. "On the contrary, he will get the approval for congress and then the military action will have additional credibility," said Qassem Saadeddine.
"Just because the strike was delayed by few days doesn't mean it's not going to happen," he said.
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 US 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 defence 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 defence battery in the Tel Aviv area to protect civilians from any possible missile attack from next-door Syria or any of its allies.
Missile defences 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.
- AP
