In harsh, uncompromising language, Secretary of State John Kerry began laying out the U.S. case for possible military action against Syria, saying there was undeniable evidence that chemical weapons had been used in a deadly attack against a rebel enclave and that it was "a moral obscenity."
Obama administration planning centers on carrying out any U.S. and allied strikes on Syria as part of a coalition without United Nations backing, U.S. and European officials said. Such a route could raise international law concerns but would let the administration avoid a potentially protracted diplomatic fight at the U.N. with Russia, President Bashar al-Assad's main backer on the Security Council. The U.S. has stepped up contacts with its North Atlantic Treaty Organization partners and the Arab League about supporting such an operation.
The U.S.'s stepped-up public rhetoric and war planning laid the groundwork for President Barack Obama to make a swift decision on launching airstrikes, even as administration officials made clear they are still awaiting the results of a final U.S. intelligence assessment on alleged chemical attacks last week that activists and rebels say killed more than 1,000 Syrians.
For now, senior administration officials said the U.S. has concluded there is "no doubt" chemical weapons were used in the incident. The administration said the evidence leaves "little doubt" that forces loyal to Mr. Assad were responsible for using the chemical weapons. U.S. intelligence agencies are now in the process of firming up those conclusions, officials said.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity," Mr. Kerry said in Washington, saying Damascus's delays in allowing international monitors to reach sites of last week's alleged attacks indicated it had something to hide, and saying that the U.S. and its allies are "actively consulting" on how to respond.
"President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people," Mr. Kerry said.
His statement came after U.N. inspectors faced gunfire Monday from unidentified snipers as they investigated reports of a chemical-weapons attack last week in the Damascus suburb of Mouadhamiya, one of the areas allegedly struck last week in poison-gas attacks.
The U.S. had earlier delivered a caution to U.N. Secretary-General Ban Ki-moon, with a senior official telling him the inspection mission was pointless and no longer safe, said a person familiar with the matter. Mr. Ban ordered his team to continue their work, this person said.
The U.N. investigators are mandated to determine whether chemical attacks occurred, but not who initiated them. U.S. officials said Monday they expected their own intelligence assessment on the attacks, details of which could be released publicly as early as Tuesday, to conclude that forces loyal to Mr. Assad were behind the poison-gas attack, not the rebels, as the Assad regime and Russia have alleged.
Administration officials made clear Mr. Obama would make his decision based on the U.S. assessment and not the findings brought back by the U.N. inspectors.
The U.S. evidence includes an analysis by U.S. spy agencies of the type of rocket used in last week's assaults to deliver chemical weapons. The agencies concluded that the type of rocket used was solely in the possession of regime forces, not the opposition, providing the White House with greater certainty of Mr. Assad's involvement, according to U.S. officials.
Pentagon officials said potential regime targets have been identified and commanders are awaiting a green light from Mr. Obama, underlining the speed with which the U.S. could act against Mr. Assad.
The U.S. Navy's Sixth Fleet has four warships in the eastern Mediterranean awaiting orders, Navy officials said. They are equipped with Tomahawk missiles and other weapon systems that can reach across Syria.
Options under consideration by the White House call for using long-range cruise missiles to take out Syrian military and intelligence command and control sites and other regime targets, U.S. officials said.
The goal of such strikes, a senior defense official said, would be to "deter and degrade" Mr. Assad's regime by raising the price for chemical weapons use and making it harder for his forces to deploy them in the future.
The U.S. warships are being kept a "healthy distance from the coast" as a precaution against Mr. Assad's advanced Russian-made coastal defenses, which include recently upgraded Yakhont missiles, a senior defense official said. U.S. officials discount the possibility that Mr. Assad might try to target U.S. warships because they are out of reach and because doing so could trigger a more devastating American response.
Administration and defense officials described the potential strikes as limited in scope, saying the goal would be to send a message to Mr. Assad without attempting to remove him.
Approval for strikes from NATO, should the U.S. seek formal backing, would require a member consensus.
There are a range of reasons for a limited response. The administration has little appetite for a protracted fight, so it is drawing a distinction between strikes aimed at the use of chemical weapons and other efforts to strengthen the Syrian opposition.
In addition, the U.S. doesn't want Mr. Assad to lose control of the chemical weapons because of the danger that they could fall into the hands of extremists. Moreover, the fall of Mr. Assad could give al Qaeda greater sway over large tracts of the country, say officials who favor only limited strikes to punish him for chemical weapons use.
For more than two years, Mr. Obama avoided U.S. military involvement in Syria's civil war. But his position has hardened considerably in response to last week's incident.
A major concern for Mr. Obama is whether U.S. inaction could embolden Mr. Assad to use chemical weapons again on a wide scale, despite Mr. Obama's declaration last year that doing so would cross his "red line."
From a strategic standpoint, advocates of limited strikes on government targets say such action is needed to burnish the Obama administration's credibility should it threaten to use military action in the future.
"He absolutely needs to act," said Aaron David Miller, a former Middle East negotiator at the State Department now at the Woodrow Wilson International Center for Scholars. "Now everyone says 'no' to the United States without cost or consequence."
Others said that Mr. Obama needs to go beyond cruise-missile strikes. "Simply taking reprisal action to say 'We mean it' does not strike me as significant meaningful action," said Anthony Cordesman, a longtime military analyst at the Center for Strategic and International Studies. "It's a pointless punitive military exercise."
White House spokesman Jay Carney made clear that the question wasn't whether Mr. Obama would respond, but how and when. "The president and his team believes that there needs to be a response that reflects the seriousness of this transgression," he said.
Once he has made that decision, he will address the American people, Mr. Carney said. The White House also began contacting lawmakers, part of a consultative process sought by members of Congress.
Late Tuesday, the State Department postponed talks with Russia that had been scheduled for later this week on a proposed peace conference on Syria, reflecting the administration's focus on the chemical weapons incident.
In addition to the rockets, officials said tissue samples extracted from the attack scenes and analyzed by the U.S. and its allies provide evidence that chemical weapons were used.
In addition to its assessment that the Assad regime is the only entity in Syria capable of carrying out such a large-scale attack, the official said, the U.S. is also tracking the regime's chemical-weapons stockpiles. That effort has provided evidence connecting the regime to this attack, the official said.
The U.N. confirmed that its investigators had come under fire Monday morning as they set out to investigate reports of a chemical-weapons attack last week in Mouadhamiya. The U.N. team turned back. Later in the day, they made it to two hospitals, interviewed survivors and doctors, and collected samples, Mr. Ban said in a statement.
In activist videos posted Monday, U.N. investigators could be seen at one field hospital wearing their signature bright blue helmets and bulletproof vests, hovering above patients being treated for exposure to the suspected chemical weapons.
"What was your location?" one U.N. inspector asked a gaunt-looking male patient seemingly in his 40s.
"I was in Al Rawda mosque," the man replied.
"What did you feel?" the inspector probed.
"It was [about] a minute and then I passed out," the patient replied, to which the translator added he had "convulsions upon his arrival."
The American message to Mr. Ban was that the U.S. believed there wasn't adequate security for the U.N. inspectors to visit the affected areas to conduct their mission, a senior administration official said. The administration also told the U.N. that the U.S. didn't think the inspectors would be able to collect viable evidence owing to the passage of time and damage from subsequent shelling, this person said. The U.N. has said such evidence would still exist.
Western governments joined Mr. Kerry in taking an increasingly stern line against Damascus.
"The suspected large-scale use of poison gas breaks a taboo even in this Syrian conflict that has been so full of cruelty," Chancellor Angela Merkel's spokesman, Steffen Seibert, said Monday. "It's a serious breach of the international Chemical Weapons Convention, which categorically bans the use of these weapons. It must be punished; it cannot remain without consequences."
The U.K. said it is "clear" that the Assad regime was behind last week's attack. British Prime Minister David Cameron cut short his holiday in order to return to London for a U.K. National Security Council meeting that has been called for Wednesday