At the Vatican, Pope Francis condemned the use of chemical weapons and appealed to all sides in Syria's civil war to lay down their arms. Francis said he would lead a worldwide prayer vigil for peace in Syria on Sept. 7.
Kerry spoke a day after President Barack Obama announced unexpectedly he would seek congressional approval before taking military action against President Bashar Assad's regime over a suspected chemical weapons attack on rebel-held areas near Damascus on Aug 21.
The decision marked a stark turnabout for the White House, which had appeared on the verge of ordering U.S. forces to launch a missile attack against Syria.
Syria's state-run news agency SANA quoted Assad saying that his government is capable of confronting a U.S. strike, but did not carry his exact comments. Assad said earlier this week that Syria "will defend itself" against Western military strikes.
Syria's deputy foreign minister, Faysal Mikdad, claimed Sunday that Obama stepped back from his threat because his administration lacks evidence of Syrian government involvement in purported poison gas attacks.
"The hesitation and the disappointment is so obvious in the words of President Obama yesterday," Mikdad told reporters in Damascus. "The confusion was clear as well."
The Assad regime alleges the Aug. 21 attacks were carried out by rebel fighters, but has not presented proof.
The state-run Syrian daily Al-Thawra, striking a gloating tone, said Obama's decision signaled defeat.
"Whether the Congress gives the red or green light for an aggression, and whether the prospects of war have been enhanced or faded, President Obama has announced yesterday, by prevaricating or hinting, the start of the historic American retreat," wrote the daily, which expresses the Syrian government's thinking.
Kerry said in appearances on several television news shows Sunday that Obama has the right to take action against Syria, with or without Congress' approval.
But he stopped short of saying Obama was committed to such a course even if lawmakers refuse to authorize force. Congress is to return from a summer break Sept. 9.
Kerry maintained there is no weakness in Obama's about-face. "This case (for an attack) is going to build stronger and stronger," Kerry told NBC's "Meet the Press."
He said that "the people of America should be celebrating that the president is not acting unilaterally."
Kerry told CNN's "State of the Union" that hair and blood samples from victims in eastern Damascus have "tested positive for signatures of sarin."
Kerry said the samples were provided to the U.S., and did not come from U.N. chemical weapons experts.
The experts spent a week in Syria, collecting biological and soil samples from stricken areas. U.N. chief Ban Ki-moon said he would present the findings as quickly as possible.
The pope, meanwhile, abandoned the traditional religious theme of the weekly papal appearance to crowds in St. Peter's Square on Sunday and instead spoke entirely, and with anguish, about Syria.
"My heart is deeply wounded by what is happening in Syria and anguished by the dramatic developments" on the horizon, Francis said, in an apparent reference to the possibility of a U.S. and French military strike.
Francis reiterated appeals for all sides in the civil war to put down their arms and "listen to the voice of their conscience and with courage take up the way of negotiations."
Over the past week, the U.S. Navy moved warships into the eastern Mediterranean as the Obama administration considered its options. Obama chose to get the backing of Congress before launching strikes, saying he believes taking that path will make the U.S. "stronger."
The White House has sent Congress a draft of a resolution seeking approval for a military response to "deter, disrupt, prevent and degrade" the Assad regime's ability to use chemical weapons. The Senate will hold hearings next week so a vote can take place after Congress gets back to work.
The president's strategy carries enormous risks to his and the nation's credibility, which the administration has argued is on the line in Syria. Obama long ago said the use of chemical weapons was a "red line" that Assad would not be allowed to cross with impunity.
British Prime Minister David Cameron charted a similar course last week by asking the House of Commons to support military action against Syria, only to suffer a stinging defeat.
Across the Atlantic, Obama's speech sparked calls for French President Francois Hollande, who supports an armed response against Syria, to seek parliamentary approval before taking military action. Hollande is not constitutionally required to do so. France's parliament is scheduled to debate the issue Wednesday, but no vote is scheduled.
For some in Syria's opposition who had put great hope in U.S. strikes, Obama's decision to postpone proved a source of despair and prolonged the torment of when - and if - Washington will act.
"Obama's speech yesterday made us feel worthless," said 29-year-old Damascus resident Nasib, who gave only his first name for fear of reprisals.
"The government here doesn't care, they're genuinely not scared, they're not gloating. So it's only provocative to us people who sit here scared, not knowing when to expect the strike," he said. "I had to tape my windows so they wouldn't break. I know people who prepared sleeping pills to give to their kids the night of the attack so they can sleep and not be scared."
For others, Obama's choice was seen as simply business as usual from a country that they say has done nothing to halt the massive trauma and bloodshed gripping Syria.
"We weren't putting too much hope in the U.S strike," said Mohammed al-Tayeb, an opposition activist in Eastern Ghouta. "America was never a friend of ours, they're still an enemy."
In the buildup to the potential strikes, the opposition and Damascus residents say the Assad regime moved it troops and military equipment out of bases to civilian areas.
The main Western-backed opposition group, the Syrian National Coalition, said in a statement Sunday that the army repositioned rocket launchers, artillery and other heavy weapons inside residential neighborhoods in cities nationwide.
Two Damascus residents confirmed the regime troop movements in interviews with The Associated Press. One woman said soldiers had moved into a school next to her house and she was terrified.
With U.S. and French strikes no longer looming, the U.N. probe into the attack has more time to analyze samples it took during on-site investigations before the specter of military action returns.
The head of the U.N. chemical experts' team, Swedish professor Ake Sellstrom, is to brief Ban later Sunday.
The inspectors left Syria on Saturday and arrived in The Hague, Netherlands. The samples they collected in Syria are to be repackaged and sent to laboratories around Europe to check them for traces of poison gas. The U.N. says there is no specific timeline for when their analysis will be completed.
There are widely varying death tolls from the suspected toxic gas attack. The aid group Doctors Without Borders says at least 355 people were killed, while the Britain-based Syrian Observatory for Human Rights monitoring groups says it has identified 502 victims by name. A U.S. intelligence assessment says the attack killed 1,429 civilians, including more than 400 children.
In Cairo, Arab League foreign ministers were to hold an emergency session Sunday evening to discuss Syria. Last week, the 22-nation bloc condemned the Aug. 21 attack outside Damascus but said it does not support military action without U.N. consent.
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Lucas reported from Beirut. Associated Press writers Yasmine Saker and Karin Laub in Beirut contributed to this report.
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