(CNN) -- The Obama administration pushed forward Sunday on a new path toward military action in Syria, urging Congress to support the president's call.
Announcing that evidence collected independently of a United Nations probe shows Syria used sarin gas in an attack on its people, U.S. Secretary of State John Kerry said the United States has to act.
"If you don't do it, you send a message of impunity," he said in an interview with CNN's "State of the Union."
Iran, North Korea, and Hezbollah "will look at the United States and say 'Nothing means anything' -- that's what's at stake here," he said.
Syria denies using chemical weapons on its people, and blames the rebels.
Even as Kerry called the evidence "overwhelming" Sunday, the United Nations argued that world leaders should wait until U.N. investigators determine whether chemical weapons were used.
"The U.N. mission is uniquely capable of establishing in an impartial and credible manner the facts of any use of chemical weapons," Martin Nesirky, spokesman for Secretary-General Ban Ki-moon, said repeatedly at a news conference.
So when will the results be ready? The U.N. won't give a timeline, Nesirky said. "It's being done as fast as it is possible to do within the scientific constraints."
Samples will be delivered to laboratories Monday, he said.
The investigation involves a strict chain of custody and clear guidelines, he said, adding that two Syrian government officials monitored the process.
But even when it's done, the U.N. will only say whether chemical weapons were used -- not who was responsible.
Obama changes course
U.S. military action appeared imminent until Saturday, when Obama announced he would first seek Congress' approval.
Lawmakers come back from recess September 9.
While some praised the president for giving Congress a chance to weigh in, Obama's decision quickly drew criticism from many on both sides of the debate over whether to strike Syria.
A key group of Syrian dissidents said it was surprised and concerned by Obama's new approach.
"We can't understand how you can promise to help those who are being slaughtered every day in the hundreds, giving them false hope, then change your mind and say let's wait and see," the Syrian National Coalition said.
Iran, a staunch supporter of the Syrian regime, warned the United States will pay a price if it strikes Syria.
Even the slightest attack by the United States against Syria will result in dire consequences, Iranian Brig. Gen. Mohammad Reza Naqdi said, according to the state-run FARS news outlet. The agency did not specify what those consequences would be.
The shift to ask for approval from Congress left some analysts scratching their heads.
"The decision-making has been so confused and muddled that it's difficult to put the word 'wise' in front of anything they're doing right now," CNN's Fareed Zakaria said on Sunday. "The administration has hesitated between nonintervention and intervention, and it is caught between those two."
The Obama administration, Zakaria said, "seems to want to have it both ways, but it can't."
Others praised the president for taking a step to get more buy-in at home and abroad.
"Frankly, I think he looks prudent, and I don't doubt his resolve on this," John Negroponte, who served as director of national intelligence for two years under President George W. Bush, told CNN's "State of the Union." "I don't think he's looking for an excuse to get out from a box or a situation that he painted himself into."
Sarin allegedly used previously in Syrian civil war
World leaders have said previously that sarin has been used in the Syrian civil war.
In April, the United States said it had evidence sarin was used in Syria on a small scale.
In May, a U.N. official said there were strong suspicions that rebel forces used the deadly nerve agent.
In June, France said sarin had been used several times in the war, including at least once by the Syrian regime.
U.S. lawmakers are deeply divided on what the United States should do after the purported chemical weapons attack on civilians August 21, which killed hundreds of people in rebel strongholds.
British intelligence had put the number of people killed in the attack at more than 350.
On Saturday, Obama said "well over 1,000 people were murdered." Kerry on Friday cited a death toll of 1,429, more than 400 of them children.
Some U.S. lawmakers have called for immediate action while others warn of stepping into what could become a quagmire.
Obama's call Saturday for congressional authorization to strike Syria surprised Washington but was applauded by members on both sides of the aisle.
White House, State Department and Pentagon officials were set to speak to members of Congress Sunday afternoon in a classified briefing.
Sen. John McCain told CBS's "Face the Nation" Sunday that Obama had invited him to a Monday meeting at the White House to discuss the next steps in Syria.
Obama's last-minute Syria switch
After signaling he was on the verge of delivering a strike against Syria, Obama made a last-minute decision Friday evening to seek congressional authorization before any military action, senior administration officials told reporters Saturday.
"While I believe I have the authority to carry out this military action without specific congressional authorization, I know that the country will be stronger if we take this course, and our actions will be even more effective," Obama said Saturday. The 1973 War Powers Act technically allows him to strike without such approval.
It's unclear how much international support the United States would have if it chooses to attack Syria.
Arab League foreign ministers were meeting in Cairo on Sunday to discuss the situation in Syria.
Britain has voted against taking any military action in Syria, and France said it won't act without the United States as a partner.
"France cannot act alone," Interior Minister Manuel Valls told Europe Radio 1 on Sunday. "There must be a coalition."
France's prime minister will meet with government ministers and other officials Monday to discuss Syria, two days before an open debate on Wednesday, the French government said.
German Foreign Minister Guido Westerwelle tweeted Sunday that the time gained waiting for U.S. congressional approval "must be used to reach a common position of the international community within the U.N. Security Council."
In another tweet, Westerwelle said the results from a U.N. inspectors' visit to Syria "must be sped up."
Amid the debate over whether to strike Syria, U.S. authorities are tightening domestic security measures. The FBI and the Department of Homeland Security are warning of a higher risk of cyberattacks after months of disruptions by hackers known as the Syrian Electronic Army, and authorities say more attacks are likely.
Syria on 'maximum readiness'
Syria's prime minister appeared unfazed by the threat of foreign intervention.
"The Syrian Army's status is on maximum readiness and fingers are on the trigger to confront all challenges," Wael Nader al-Halqi said, according Syrian state-run TV.
Maria Saadeh, a member of Syria's parliament, told CNN on Sunday that she sees no justification for a U.S. strike on Syria.
"There is no legitimacy to make this attack," she said, accusing rebel groups of using chemical weapons and committing other crimes against humanity.
The Syrian government has denied that it used chemical weapons in the August 21 attack, saying that jihadists fighting with the rebels used them in an effort to turn global sentiments against the regime.
Syrian state media have been packed with critiques of the U.S. position since Obama's announcement Saturday. An editorial in the state-run Al-Thawra newspaper Sunday said that Obama had declared "the beginning of a historic American retreat."
Meanwhile, the opposition Syrian National Coalition issued a statement aimed at pushing U.S. lawmakers to take action.
"The Syrian National Coalition calls on the American congress to carry their historical responsibility towards the Syrian people," the opposition said in the statement, "and take the right decision to support the American government approach to stop the killing machine of the Syrian criminal regime."