"We're not considering a boots-on-the-ground approach," Obama said, seeking to reassure a public weary after a decade of war in Iraq and Afghanistan.
With France as his only major ally, Obama told reporters he has a strong preference for multilateral action but "we don't want the world to be paralyzed."
Halfway around the world, U.S. warships were in place to deliver a punishing blow.
At the same time, U.N. personnel carried out a fourth day of inspection as they sought to determine precisely what happened in the attack last week. The international contingent arranged to depart Syria on Saturday and head to laboratories in Europe with the samples they have collected.
Video said to be taken at the scene shows victims writhing in pain, twitching and exhibiting other symptoms associated with exposure to nerve agents. The videos distributed by activists to support their claims of a chemical attack were consistent with AP reporting of shelling in the suburbs of Damascus at the time, though it was not known if the victims had died from a poisonous gas attack.
Residents of Damascus stocked up on food and other necessities in anticipation of strikes, although no signs of panic or shortages were evident.
"We got used to the sound of shelling" after three years of civil war, said Kheireddine Nahleh, a 53-year-old government employee. "Death is the same, be it with a mortar or with an American missile. I'm not afraid."
Obama met with his national security aides at the White House and then with diplomats from Baltic countries, saying he has not yet made a final decision on a response to the attack.
But the administration did nothing to discourage the predictions that he would — and soon. It was an impression heightened both by strongly worded remarks by Secretary of State John Kerry's remarks and the release of an unclassified intelligence assessment that cited "high confidence" that the Syrian government carried out the attack.
In addition to the dead, the assessment reported that about 3,600 patients "displaying symptoms consistent with nerve agent exposure" were seen at Damascus-area hospitals after the attack. To that, Kerry added that "a senior regime official who knew about the attack confirmed that chemical weapons were used by the regime, reviewed the impact and actually was afraid they would be discovered." He added for emphasis: "We know this."
The assessment did not explain its unexpectedly large casualty count, far in excess of an estimate from Doctors without Borders.
Mindful of public opinion, Kerry urged Americans to read the four-page assessment for themselves. He referred to Iraq — when Bush administration assurances that weapons of mass destruction were present proved false, and a U.S. invasion led to a long, deadly war. Kerry said this time it will be different.
"We will not repeat that moment," he said.
Despite difficulty in rounding up foreign allies, Defense Secretary Chuck Hagel, travelling in the Philippines, said, "Our approach is to continue to find an international coalition that will act together." So far, only France has announced it will support military action.
U.S. warships were in place in the Mediterranean Sea armed with cruise missiles, long a first-line weapon of choice for presidents because they can find a target hundreds of miles distant without need of air cover or troops on the ground.
White House officials previewed the intelligence findings Thursday night in a telephone briefing for senior members of Congress.
The Assad government has accused rebels of carrying out the attacks.
The looming confrontation is the latest outgrowth of a civil war in which Assad has tenaciously — and brutally — clung to power. An estimated 100,000 civilians have been killed in more than two years, many of them as a result of attacks by the Syrian government on its own citizens.
Obama has long been wary of U.S. military involvement in the struggle, as he has been with turbulent events elsewhere during the so-called Arab Spring. In this case, reluctance stems in part from a recognition that while Assad has ties to Iran and the terrorist group Hezbollah, the rebels seeking to topple him have connections with al-Qaida terrorist groups.
Still, he declared more than a year ago that the use of chemical weapons would amount to a "red line" that Assad should not cross. And Obama approved the shipment of small weapons and ammunition to the Syrian rebels after an earlier reported chemical weapons attack, although there is little sign that the equipment has arrived.
With memories of the long Iraq war still fresh, the political cross-currents have been intense both domestically and overseas.
Dozens of lawmakers, most of them Republican, have signed a letter saying Obama should not take military action without congressional approval, and top leaders of both political parties are urging the president to consult more closely with Congress before giving an order to launch hostilities.
Despite the urgings, there has been little or no discussion about calling Congress back into session to debate the issue. Lawmakers have been on a summer break for nearly a month, and are not due to return to the Capitol until Sept. 9.
Obama's efforts to put together an international coalition to support military action have been more down than up.
French President Francois Hollande has endorsed punitive strikes, and told the newspaper Le Monde that the "chemical massacre of Damascus cannot and must not remain unpunished."
But British Prime Minister David Cameron's attempt to win a vote of approval in Parliament for military action ended in ignominious defeat on Thursday. American attempts to secure backing at the United Nations have been blocked by Russia, long an ally of Syria.
United Nations Secretary General Ban ki-moon has urged a delay in any military action until the inspectors can present their findings to U.N. member states and the Security Council.
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