Secretary of State John Kerry condemned Syria for mass killing. NBC News’ Andrea Mitchell reports.

Making a forceful case to answer a “crime against conscience,” Secretary of State John Kerry declared Friday that the U.S. had a moral obligation to punish Syria for using chemical weapons — painting a ghastly portrait of twitching bodies, victims foaming at the mouth and row upon row of children gassed to death.

He called Syrian President Bashar al-Assad “a thug and a murderer” and pledged, to a country weary after more than a decade of war in the Middle East, that the U.S. response wouldn’t require ground troops and wouldn’t be open-ended.

The U.N. team of chemical weapons inspectors collecting samples from the site of the alleged attack wrapped up its work and pulled out of the country ahead of schedule early Saturday as it became clearer that President Barack Obama might be prepared to strike Syria unilaterally, in the face of objections from allies and many in Congress.

Obama said he hadn’t made a decision on military action, but he echoed Kerry in saying any U.S. action would be limited. The U.S. has an obligation “as a leader in the world” to hold foreign nations to account when they use prohibited weapons, Obama said.

“This kind of offense is a challenge to the world,” Obama said after a meeting with Baltic leaders. While the U.S. would prefer to act with the broad support of the international community, which has so far not been forthcoming, Obama said, “we don’t want the world to be paralyzed.”

An administration official confirmed that Obama spoke Friday with British Prime Minister David Cameron and French President Francois Hollande.

With Americans skeptical, members of Congress raising questions and other nations objecting to a U.S. attack outright, Kerry said the world had to answer what he called a crime against humanity itself, carried out last week in the suburbs of Damascus, the Syrian capital. He said the attack killed 1,429 people, including 426 children.

Secretary of State John Kerry explains why military action against Syria would be justified. Watch his full address.

“Instead of being tucked safely in their beds at home,” he said, “we saw rows of children lying side by side, sprawled on a hospital floor, all of them dead from Assad’s gas and surrounded by parents and grandparents who had suffered the same fate.”

He declared: “My friends, it matters here if nothing is done. It matters if the world speaks out in condemnation and then nothing happens.”

Referring directly to the long wars in Iraq and Afghanistan, he said: “Fatigue does not absolve us of our responsibility. Just longing for peace does not necessarily bring it about.”

As he spoke, the White House released an intelligence report (.pdf) claiming “high confidence” that the Syrian regime had used chemical weapons. The report cited a “large body of independent sources” but acknowledged that not all the evidence could be declassified.

Syrian TV quoted an official source at the Ministry of Foreign Affairs and Emigrants as denying every point in Kerry’s speech, dismissing them as “some old stories the terrorists fabricated more than a week ago — totally full of lies and fabrications.”

“The figures put forward by Mr. Kerry are absolutely fake numbers, and the source is the insurgents in Syria and external opposition that incite U.S. aggression,” said the report, which was translated by NBC News. It said Kerry’s speech should “remind the world of the lies fabricated and delivered by [then-Secretary of State] Colin Powell before the invasion of Iraq.”

Just before Kerry spoke, the U.N. said its inspectors had finished collecting samples from the site of the attack. But it said a complete analysis would take time and offered no sense of when it would be complete. Secretary-General Ban Ki-moon told the five permanent Security Council members that it may be two weeks before final results are ready, diplomats told Reuters.

Obama on his own

A final war plan is in place, but President Barack Obama hasn’t yet decided what action to take in Syria. NBC News’ Jim Miklaszewski reports.

Britain, the most steadfast U.S. ally, rejected military action in a stunning vote Thursday night. Acting through the U.N. is a dead end because China and Russia, which have veto power in the Security Council, will not allow it.

And at home, members of Congress have insisted that Obama get lawmakers’ approval. Americans appear to agree: In a poll released Friday by NBC News, almost 8 in 10 said they wanted the president to sell Congress on military action before an attack.

The poll found that support among Americans is higher for a limited military strike, such as the firing of cruise missiles from Navy warships in the Mediterranean Sea. But half of Americans are opposed to any military attack.

White House officials told NBC News that the administration was prepared for the U.S. to go it alone. Still, even after a briefing from the administration officials, some members of Congress were unconvinced.

Rep. Buck McKeon, R-Calif., chairman of the Armed Services Committee, said he was disappointed that Obama himself didn’t take part.

“One of the things that has really bothered me is the president drawing a red line without knowing in his mind what he would do if they crossed the red line,” he said, referring to a remark Obama made a year ago about Syria’s potential use of chemical weapons.

Other lawmakers, including Sens. Bob Corker, R-Tenn., and Robert Menendez, D-N.J., emerged from the briefing persuaded, NBC News reported.

Late Friday, the White House scheduled another briefing for Republican senators  Saturday at the request of Senate Republican leader Mitch McConnell of Kentucky.

“Sen. McConnell believes it’s important for the whole Conference to have the opportunity to communicate directly with the administration on this important issue,” McConnell’s office said.

A Syrian official told NBC News officials the government was expecting a U.S. attack.

“We know it’s going to come. We just don’t know when,” the official said.

Richard Engel, Andrea Mitchell, Kelly O’Donnell, David Rudge, Shawna Thomas, Frank Thorp, Kristen Welker, Matthew DeLuca and M. Alex Johnson of NBC News contributed to this report.

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