President Obama has the right to order a strike on Syria, officials insisted, as allies pulled back and U.S. lawmakers sought a congressional vote on the issue.
"President Obama's decision-making will be guided by what is in the best interests of the United States," White House National Security Council spokeswoman Caitlin Hayden said.
"He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable," she said.
The Pentagon said the Navy now had a fifth destroyer in the eastern Mediterranean Sea. Each ship carries dozens of Tomahawk long-range cruise missiles.
Officials have said an attack would likely involve guided missiles launched from the sea and possibly from bomber aircraft.
Obama lost military support from staunch ally Britain Thursday after Prime Minister David Cameron was defeated in a House of Commons vote on whether to endorse military action.
Cameron just days ago called for punishing Syrian President Bashar Assad's forces for alleged use of chemical weapons against rebel-held areas outside Damascus Aug. 21. That attack killed hundreds of civilians.
Britain "will not be involved" in any military strikes on Syria, Defense Secretary Philip Hammond said after the vote.
But he added, "I don't expect that the lack of British participation will stop any action."
French President Francois Hollande, who earlier insisted Assad must be militarily "punished," and German Chancellor Angela Merkel, who also offered support, both called for a delay in any military operation until the U.N. Security Council can review evidence collected by chemical weapons inspectors now in Syria.
U.N. Secretary-General Ban Ki-moon told reporters in Vienna the 20-member team would complete its work Friday and leave Syria Saturday.
Obama is scheduled to leave for an overseas trip Tuesday. That trip, to Russia and the Group of 20 summit, is expected to keep him away from the White House for the rest of the week.
Ban said he expected an "immediate" report from the weapons inspectors. He didn't say how many days "immediate" meant.
The inspection team's mission is to determine if a chemical attack occurred, not to assign blame.
Top U.S. State Department, Pentagon and intelligence agency officials sought Thursday evening to persuade Republican and Democratic lawmakers of both chambers the evidence against Assad was clear.
Among the officials on a 90-minute, unclassified conference call were Secretary of State John Kerry, Defense Secretary Chuck Hagel, national security adviser Susan Rice, National Intelligence Director James Clapper and Joint Chiefs of Staff Vice Chairman Navy Adm. James A. Winnefeld Jr.
The officials didn't present intelligence that directly tied Assad to the attack, but they said they had enough evidence to be highly confident his forces carried it out and his regime was therefore responsible, said lawmakers and officials who participated in the call.
A critical piece of the intelligence presented on the call was an intercepted phone call between Syrian military officials discussing the gas attacks, several participants said.
One Syrian military official seemed to suggest the chemical weapons attacks were more devastating than intended, an intelligence official said.
"It sounds like he thinks this was a small operation that got out of control," the intelligence official told The New York Times.
But Republican lawmakers said White House officials dismissed any thought the attack's scale was miscalculated, indicating the administration believes Syria intended to inflict the widespread damage.
Rep. Eliot Engel, D-N.Y., said after the call he believed administration officials presented "more than enough evidence" for Obama to act if he chooses to.
But House Armed Services Committee Chairman Buck McKeon, R-Calif., said Obama had not yet made a forceful-enough case to persuade both Congress and the American people.
More than 150 House members from both parties have signed a letter to Obama calling on him to seek formal congressional approval for any military action. The number of lawmakers signing the letter was 116 Wednesday.
Sen. Bob Corker, R-Tenn., the senior Republican on the Senate Foreign Relations Committee, was quoted by The Wall Street Journal as saying he'd support "surgical, proportional military strikes, given the strong evidence of the Assad regime's continued use of chemical warfare."
Sen. Lindsey Graham, R-S.C., who took part in a separate briefing by White House Chief of Staff Denis McDonough, said he was also comfortable Assad bore responsibility for the chemical attacks.
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