Secretary of State John Kerry said Sunday that the United States has obtained fresh evidence that sarin nerve gas was used in a deadly attack in Syria, as he sought to bolster the case for taking military action against the government of President Bashar al-Assad.

"Let me just add that this morning, a very important recent development, that in the last 24 hours, we have learned through samples that were provided to the United States that have now been tested from first responders in east Damascus and hair samples and blood samples have tested positive for signatures of sarin," Kerry said in an interview with "Meet The Press," according to a transcript provided by NBC News. "So this case is building and this case will build."

Kerry is appearing on all five Sunday news shows a day after President Obama said the U.S. should take military action against Syria for what he has concluded is a chemical attack. But Obama said he would first seek authorization from Congress for a strike. "This attack is an assault on human dignity," Obama said in remarks Saturday at the White House.

In a Friday speech, Kerry said U.S. intelligence has "high confidence" the Syrian government was responsible for an Aug. 21 attack on the outskirts of Damascus. The White House released an intelligence assessment that same day as it sought to make its case the American people. The attack, Kerry said, killed at least 1,429 Syrians.

Lawmakers are in recess until Sept. 9. The debate over Syria could spark intense debate in Congress, where sharply contrasting views over foreign policy and national security have been laid bare in recent months.

Kerry defended Obama's decision to seek authorization from Congress in an interview with CNN. He said that Obama "has the right" to launch a strike even without Congress's approval, but that there is great value in seeking the input of lawmakers.
"Sometimes, the wheels of democracy require us to take the extra day or two to provide the legitimacy that our founding fathers contemplated in actions that we take," Kerry said on "State of the Union."
Kerry also responded to critics, saying that people shouldn't have ever jumped to conclusions about what Obama would do.
"Just because he disappointed some people who thought, without any basis, that he was setting up to go take a strike doesn't mean he didn't reserve the right to make the judgement that he made," Kerry said.