A senior administration official said the president's remarks were not about imminent military operations in Syria, but rather an update about how his decisions on how to proceed. The official declined to be identified ahead of the president's speech.

The Obama administration was to continue its sales pitch to Congress to get behind a potential military strike against Syria on Saturday, hours after U.N. weapons inspectors left the country amid high anticipation of an imminent U.S. attack.

The president's national security team agreed to brief the entire Senate Republican conference on Syria at the request of Senate Minority Leader Mitch McConnell, according to the Kentucky lawmaker's spokesman, Don Stewart.

The Saturday afternoon telephone briefing, set to begin at 1 p.m. ET, follows a classified briefing for some lawmakers by Secretary of State John Kerry, National Security Adviser Susan Rice, Defense Secretary Chuck Hagel and Director of National Intelligence James Clapper about the White House's intelligence on Syria's chemical use.

Also on Saturday, the office of House Speaker John Boehner announced that the White House plans to provide a classified briefing at 2 p.m. Sunday to members who wished to attend.

The briefings will come hours after U.N. experts, who had been collecting samples from last week's alleged chemical weapons strike outside Damascus, left the country bound for the Netherlands.

The chemical weapons experts were working to determine what occurred in the apparent chemical weapons attack near Damascus on Aug. 21, which U.S. intelligence reports say left 1,429 people dead, including 426 children. They have taken blood and urine samples from victims and soil samples from areas where chemical attacks have been reported. The samples will be tested in Europe.

Russian President Vladimir Putin urged President Obama on Saturday not to rush into a decision on striking Syria.

"If there is evidence, it should be presented," Putin said. "If it is not presented, that means it does not exist."

Putin's comments come after Obama said Friday that he is considering a "limited, narrow act" as a military response to Syria's alleged use of chemical weapons against its own citizens.

Calling it "a challenge to the world," Obama said the use of chemical weapons threatens U.S. national security and merits a response.

"We're not considering any open-ended commitment," the president said. "We're not considering any boots-on-the-ground approach."

Kerry also spoke Friday, detailing the intelligence community's findings and announcing the release of a four-page report summarizing the administration's case against Syrian President Bashar Assad's regime.

"I'm not asking you to take my word for it," Kerry told reporters at the State Department. "Read for yourselves the verdict reached by our intelligence community" that the government of Syria was responsible for the attack.