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Obama will seek congressional approval before any military action against Syria

President Barack Obama will seek authorization from Congress before launching any military action against the Syrian regime for allegedly using chemical weapons in a mass killing that the U.S. says claimed the lives of 1,429 people.The president came to the unexpected decision during a walk Friday evening with his chief of staff Denis McDonough, just hours after Secretary of State John Kerry made

President Barack Obama will seek authorization from Congress before launching any military action against the Syrian regime for allegedly using chemical weapons in a mass killing that the U.S. says claimed the lives of 1,429 people.

The president came to the unexpected decision during a walk Friday evening with his chief of staff Denis McDonough, just hours after Secretary of State John Kerry made a forceful case for the U.S. to attack Syria, sources told NBC News. After Obama returned from the stroll around the South Lawn, he called senior aides, leading to meetings Friday night and Saturday morning, the sources said.

Obama stressed on Saturday that American warships in the Mediterranean Sea still stood poised to strike regime targets at any time, despite the move that would place a hold on any imminent military action.

"Over the last several days, we have heard from members of Congress who want their voices to be heard," Obama said. "I absolutely agree."

"We should have this debate, because the issues are too big for business as usual," he said.

Hours later, he sent congressional leaders draft legislation requesting "authorization for the use of United States Armed Forces in connection with the conflict in Syria."

Just minutes after Obama's statement, the Syrian army recommenced its shelling of rebel-held Damascus suburbs, which had halted for several hours. 

In his statement, Obama condemned Assad's regime, describing the alleged chemical attack as "an assault on human dignity" that "presents a serious danger to our national security." He had previously characterized the use of chemical weapons as a "red line" that Assad should not be permitted to cross.

Obama pledged that any military involvement would be of "limited duration and scope."

"This would not be an open-ended intervention," he said. "We would not put boots on the ground."

Before saying Saturday that he would seek approval from Congress, the President made clear that "we are prepared to strike whenever we choose." 

"Our capacity to execute this mission is not time-sensitive," Obama said. "It will be effective tomorrow or next week or one month from now, and I am prepared to give that order."

And yet many Syrian rebel groups expressed dismay with the apparent postponement of military intervention. One rebel spokesman told NBC News: “President Obama is sending contradictory messages. He promised to help, and now promises delays.”

“If Congress votes against a military action,” the spokesman said, “it will mean the American people don’t want to help the Syrian people.”

Kerry on Saturday afternoon spoke with Syrian Opposition Coalition President Ahmed Assi al-Jarba to “underscore President Obama’s commitment to holding the Assad regime accountable,” according to a statement from a senior State Department official.

WATCH: Richard Engel: For Syrians, 'this is absolutely a time-sensitive issue'

Key Republican leaders, including House Speaker John Boehner, R-Ohio, and Majority Leader Eric Cantor, R-Va., issued a joint statement Saturday afternoon applauding the president’s decision to take his case for intervention to their official chambers.

“Under the Constitution, the responsibility to declare war lies with Congress,” they said. “We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised.”

Congress is slated to return from its five-week summer recess on Sept. 9. But Senate Majority Leader Harry Reid, D-Nev., said the upper chamber would begin hearings on the issue next week, ahead of what he pledged would be a vote no later than the week of Sept. 9th.

Meanwhile, citizens in Syria worried Saturday about the skyrocketing price or sudden absence of food, gasoline, and medical supplies. The city was checkered by roadblocks, and traffic was slowed to a crawl.

Activists in the Syrian capital told NBC News that people in some Damascus neighborhoods waited seven hours in bread lines, and scooped up other essential items like rice, tea, and sugar in preparation for a strike. Some people who live in close proximity to possible targets like military installations and barracks have moved, the activists said.

WATCH: Andrea Mitchell on why Obama decided to go to Congress first

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The White House released an intelligence report as Kerry spoke on Friday claiming “high confidence” that the Syrian regime had deployed chemical weapons. The report cited a “large body of independent sources” but said that not all the evidence of the alleged attack could be declassified. 

WATCH: Protest heard within earshot of Obama's Rose Garden statement

The Obama administration faces fierce opposition from many members of the U.S. Senate and wide swaths of the American public reluctant to become entangled in a Middle Eastern conflict just a decade after the U.S. invasion of Iraq.

Prior to Kerry's remarks Friday, an NBC News poll showed that nearly 80 percent of Americans believed Obama should get a stamp of congressional approval before using force in Syria. Fifty percent of Americans believe the U.S. should not intervene at all, according to the poll.

A chorus of opposition from global partners and domestic constituents rose on Saturday as it appeared that the U.S. may be prepared to bring the confrontation to a climax.

Sen. John McCain, R-Ariz., decried Obama's plans for limited air strikes on Syria as "cosmetic" during an interview with Jay Leno on "The Tonight Show" on Friday, saying the failure of the U.S. to intervene in the country's bloody, brutal civil war was "shameful."

"The president apparently wants to have a kind of cosmetic strike, launch a few missiles and then say, 'Well, we responded.' This is the same president that, two years ago, said Bashar Assad had to go," McCain said.

Russian President Vladimir Putin, one of Assad’s stalwart allies, said the U.S. should present its evidence of the attack to the U.N. inspectors and U.N. Security Council.

The Russians hold veto power in the U.N. Security Council, which accounts for why Kerry did not take his forceful case for military intervention there. The Russian leader’s remarks were his first public comments on the unfolding crisis since the alleged Aug. 21 attack.

Britain, the most steadfast U.S. ally, rejected military action in a stunning vote Thursday night, delivering a dramatic blow to the Obama administration and thwarting any attempt to build a robust coalition of Western powers. 

However, French President Francois Hollande has publicly endorsed international military action against the Assad regime, telling the French newspaper Le Monde that the alleged chemical attack “must not go unpunished.”

Obama on Saturday afternoon informed Hollande that the U.S. is prepared to take limited military action against Syria and thanked him for “France’s principled commitment to upholding the international norm against the use of chemical weapons and enforcing the consequences that give this norm meaning,” according to a statement from the White House.

Senior U.S. officials Saturday will hold unclassified conference calls with the Senate Republican Conference and the Senate Democratic Caucus to continue consultations on potential U.S. military action, according to a White House official.

WATCH: Jim Miklaszewski on why keeping military 'on standby' could be taxing

NBC News' Andrea Mitchell, Catherine Chomiak, Madeleine Haeringer, Ghazi Balkiz, Frank Thorp, and M. Alex Johnson contributed to this report.