Secretary of State John Kerry confirmed that the United States believes Syrian President Bashar al-Assad and his government forces deployed chemical weapons against rebels during an attack last week while speaking at a press briefing Monday.
Pressure has been mounting on the Obama administration to move ahead with military action following the suspected attack, but Kerry's words marked an official change in tone.
"What we saw in Syria last week should shock the conscience of the world," he said. "It defies any code of morality. The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it was inexcusable."
By playing up the humanitarian angle, Kerry could be trying to build domestic support for increased U.S. military involvement in the region, something currently very unpopular.
Only about 9 percent of Americans say the United States should intervene in Syria, compared with 60 percent who disagree, according to a recent Reuters survey.
But Kerry's briefing made it clear the Obama administration is ready to change course following the obvious use of chemical weapons, a "red line" the president identified last year.
"While investigators are gathering additional evidence on the ground, our understanding of what has already happened in Syria is grounded in facts, informed by conscience and guided by common sense," Kerry said.
"The reported number of victims, the reported symptoms of those who were killed or injured, the first-hand accounts from humanitarian organizations on the ground, like Doctors Without Borders and the Syria Human Rights Commission, these all strongly indicate that everything these images are already screaming at us is real; that chemical weapons were used in Syria."
Kerry added that the administration was consulting with members of Congress and that Obama will be making "an informed decision" about how to respond to the chemical weapon attack.
"But make no mistake, 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," he said.