The US has said it has proof Syria used the chemical weapon sarin in last month's attacks. Syria has said it is "capable of confronting" aggression after President Barack Obama asked Congress to vote on miltary action.
US Secretary of State John Kerry told NBC and CNN television that hair and blood samples provided to the United States from the scene of last month's Damascus attacks "have tested positive for signatures of sarin."
Kerry said that the "case is building" for military action, adding that he was confident Congress "will do what is right" in an upcoming vote on US intervention.
President Obama announced on Saturday he had taken the unexpected move of submitting a bill to Congress to authorize the use of force.
"I will seek authorization for the use of force from the American people's representatives in Congress," the president said, adding that the vote would take place as soon as lawmakers return from their summer break on 9 September.
"I'm comfortable with going forward without a UN Security Council, which has so far been completely paralyzed and unable to hold [President Bashar] Assad accountable," he added.
The accelerated push for military intervention comes in response to an alleged chemical weapons attack in the Damascus suburb of Ghouta on August 21, which the US claims was carried out by the Assad regime. According to Washington, 1,429 people died in the attacks, including 426 children.
President Assad responded to the US warning on Sunday, reiterating that Syria was ready for any intervention.
"Syria ... is capable of confronting any external aggression just as it faces up to internal aggression every day, in the form of terrorist groups and those that support them," state television SANA quoted him as saying.
"The American threats of launching an attack against Syria will not discourage Syria away from its principles ... or its fight against terrorism supported by some regional and western countries, first and foremost the United States of America."
While Assad addressed Obama's call for military action, others in Syria were quick to note his unexpected decision to put it to Congress first, a turnabout for the White House which had previously appeared to be close to ordering an imminent strike.
"Obama announced yesterday, directly or through implication, the beginning of the historic American retreat," read the front page of Syrian state newspaper al-Thawra.
It wrote that Obama's decision to put military intervention to a congressional vote stems from his "implicit defeat and the disappearance of his allies," in an apparent reference to a British parliamentary vote rejecting military intervention.
Meanwhile Syria's deputy foreign minister, Faysal Mikdad described it as proof the White House had little evidence the Syrian government was behind the alleged attacks, saying Obama had become "hesitant and confused."
"The hesitation and disappointment is so obvious in the words of President Obama yesterday," Mikdad told reporters in Damascus. "The confusion was clear and clear."
He reiterated that rebel forces were behind the Damascus attacks, denying any regime involvement, and urging the US Congress to show "wisdom" in its upcoming vote.
Mikdad also labeled as "irresponsible" plans by France, the only other permanent member of the UN Security Council, to support military action.
France eyes US vote
On Sunday French Interior Minister Manuel Valls reiterated that military intervention was necessary.
"The chemical massacre at Damascus must not remain unpunished. We must bring an end to this regime," he told Europe1 radio.
He noted, however, that France did not intend to "go it alone," and would await the outcome of the US bill.
French President Francois Hollande must now weigh up whether to follow in the footsteps of the US and Britain and hold its own parliamentary vote on an armed response, something he is not legally required to do.
Stop 'killing machine,' says opposition
Among the Syrian opposition, delays invoked by turning the decision over to lawmakers, drew statements of despair. The Syrian opposition coalition issued a statement on Sunday calling on the US to urgently approve military action as well as arm the rebel Free Syrian Army.
"The National Coalition calls on members of the US Congress to assume their historic responsibility towards the Syrian people, and ... stop the regime's killing machine."
"If the free world fails to respond to such an outrageous breach of international norms, dictators around the world will be encouraged in their efforts to follow the example set by Assad," it added.
Pope calls for fasting next Saturday
According to the latest figures issued by the Britain-based Syrian Observatory for Human Rights, more than 110,000 people have been killed since fighting erupted in March 2011 amid increasing allegations of chemical attacks.
At the Vatican on Sunday, Pope Francis condemned the use of chemical weapons and announced that he would lead a worldwide day of fasting and prayer for peace next Saturday, 7 September.
"My heart is deeply wounded by what is happening in Syria and anguished by the dramatic developments," Francis said.
He reiterated previous 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."
Francis had abandoned the traditional religious theme of his weekly papal appearance to crowds in St. Peter's Square and instead spoke largely about Syria.
ccp/ipj (AFP, Reuters, AP)