President Barack Obama and Secretary of State John Kerry condemned the gassing of Syrian civilians, saying the action cannot go unanswered even as the British Parliament rejected any military involvement.
"We cannot accept a world where women and children -- innocent civilians -- are gassed on a grand scale," Obama said following meetings with Baltic leaders at the White House.
Kerry called Syrian President Bashar Assad a "murderer" and said U.S. credibility rests on what is done in response to the Aug. 21 attack in the Damascus suburbs that left more than 1,400 dead, including more than 400 children.
"A large body of independent sources indicates that a chemical weapons attack took place in the Damascus suburbs on Aug. 21," the assessment released by the White House reads. "In addition to U.S. intelligence information, there are accounts from international and Syrian medical personnel, videos, witness accounts, thousands of social media reports from at least 12 different locations in the Damascus area, journalist accounts and reports from highly credible non-governmental organizations. ...
"We assess with high confidence that the Syrian government carried out the chemical weapons attack against opposition elements in the Damascus suburbs on Aug. 21. We assess that the scenario in which the opposition executed the attack on Aug. 21 is highly unlikely."
Kerry called the use of chemical weapons a "crime against conscience ... against humanity ... against the most fundamental principles of international community, against the norm of the international community."
Obama said he has ruled out sending in troops although he has yet to decide what action to take.
"The world has an obligation that we maintain the norm against the use of chemical weapons," he said.
The U.S. assessment was bolstered by France. President Francois Hollande offered strong support for international action, saying his government was "prepared to punish" those responsible, The New York Times reported.
Hollande said the use of chemical weapons in Syria was "an established fact," adding, "France possesses a body of evidence that goes in the sense of the regime's responsibility" for the attacks.
He said a strike against Syrian government forces by international forces would have "dissuasive value."
Hollande 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.
A U.N. Security Council meeting on the situation Thursday ended without resolution.
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 bombers.
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 whether a chemical attack occurred, not to assign blame.