U.S. Secretary of State John Kerry made clear on Friday that the United States will punish Syrian President Bashar al-Assad for the "brutal and flagrant" chemical weapons attack that he said killed more than 1,400 people in Damascus last week.
Kerry said it was essential not to let Syria get away with the attack, partly as a sign to those who might consider using chemical weapons in the future, and said the United States was joined by allies including France, "our oldest ally," in its determination to act.
"It matters here if nothing is done," Kerry said in a statement delivered at the State Department. "It matters if the world speaks out in condemnation and then nothing happens."
Kerry laid out a raft of evidence he said showed Assad's forces were behind the attack, and the U.S. Government released an unclassified intelligence report at the same time including many of the details.
The report said the August 21 attack killed 1,429 Syrian civilians, including 426 children.
The intelligence gathered for the U.S. report included an intercepted communication by a senior official intimately familiar with the August 21 attack as well as other intelligence from people's accounts and intercepted messages, the four-page report said.
France said on Friday it still backed military action to punish Assad's government for the attack despite a British parliamentary vote against a military strike.
An aide to Russian President Vladimir Putin, a close Assad ally, seized on Thursday's British "no" vote which set back U.S.-led efforts to intervene against Assad, saying it reflected wider European worries about the dangers of a military response.
Any military strike looks unlikely at least until U.N. investigators report back after they leave Syria on Saturday.
The timing of any strikes may be complicated by Obama's departure late on Tuesday for Sweden and a G20 summit in Russia. He was not expected to order the strikes while in Sweden or Russia.
French President Francois Hollande told the daily Le Monde he still supported taking "firm" punitive action over an attack he said had caused "irreparable" harm to the Syrian people, adding that he would work closely with France's allies.
Britain has traditionally been the United States' most reliable military ally. However, the defeat of a the government motion authorizing a military response in principle underscored misgivings dating from how the country decided to join the U.S.-led invasion of Iraq in 2003.
Russia, Assad's most powerful diplomatic ally, opposes any military intervention in Syria, saying an attack would increase tension and undermine the chances of ending the civil war.
Putin's senior foreign policy adviser, Yuri Ushakov, said the British vote represented majority opinion in Europe.
"People are beginning to understand how dangerous such scenarios are," he told reporters. "Russia is actively working to avert a military scenario in Syria.