In a blistering statement Monday, Secretary of State John Kerry said it was clear that a large-scale chemical weapons attack had occurred in Syria and said the regime’s claims that it was not responsible lack credibility.

“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,” Kerry told reporters. “Nothing today is more serious and nothing is receiving more serious scrutiny.”

Kerry said Monday that Congress will be consulted as Obama decides what to do next.

“The administration is actively consulting with members of Congress, and we will continue to have these conversations in the days ahead,” Kerry said. (Earlier Monday, a spokesman for Speaker John A. Boehner, R-Ohio, said Obama had not yet called the speaker.)

Kerry dismissed allegations that the attack might have been faked.

“Anyone who can claim that an attack of this staggering scale can be contrived or fabricated needs to check their conscience and their own moral compass. What is before us today is real and it is compelling,” he said.

He said the Syrian regime maintains custody of its chemical weapons and has the means to deliver them with rockets. And he said an offer to let the United Nations investigate is “too late to be credible.”

Kerry accused the Syrian government of shelling the area for days after the attack and “systematically destroying evidence.”

“That is not the behavior of a government that has nothing to hide,” Kerry said.

Later Monday, White House Press Secretary Jay Carney reiterated Kerry’s comments, saying there was “very little doubt” that the Syrian regime had carried out the chemical attack and no doubt that a chemical attack had occurred. Carney said members of Congress had been consulted by the White House and would continue in the coming days. But he repeatedly declined to speculate on when the president would decide what to do and if he would seek congressional approval — only that the attack required a response.

“What we are evaluating now is a response to the clear use on a mass scale with repugnant results of chemical weapons. And there is very little doubt that the Syrian regime, the Assad regime, used those weapons, because they have maintained control of the stockpile, of chemical weapons in Syria. They alone have the capacity to use rockets to deliver chemical weapons,” Carney said.