It would essentially pit the U.S. and regional allies Saudi Arabia, Turkey and Qatar in a proxy war against Iran, which is providing weapons to the Syrian government's counterinsurgency, along with Hezbollah, the militant Lebanese group that also has aided Assad's forces militarily.
Deputy Foreign Minister Faysal Mikdad told The Associated Press in an interview in Damascus that such an attack would trigger "chaos in the entire world."
"If individual countries want to pursue aggressive and adventurous policies, the natural answer ... would be that Syria, which has been fighting against terrorism for almost three years, will also defend itself against any international attack," he added.
Assad told a Russian newspaper that any military campaign against his country was destined to fail.
It's also unclear what U.S. action would mean for relations with Russia, which warned Monday against the use of force not sanctioned by the U.N. Security Council, calling it "a crude violation of international law."
Support for some sort of international military response was likely to grow if it is confirmed that Assad's regime was responsible for the Aug. 21 attack in the Damascus suburbs that activists say killed hundreds of people. The group Doctors Without Borders put the death toll at 355.
U.S. Secretary of State John Kerry said chemical weapons were used in Syria, and he accused Assad's regime of destroying evidence. "The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable and despite the excuses and equivocations that some have manufactured it is undeniable," said Kerry, the highest-ranking U.S. official to confirm the attack.
"This international norm cannot be violated without consequences," he said.
Assad has denied launching a chemical attack, blaming the rebels instead, and has authorized a U.N. team of experts in Syria to investigate, although the U.S. said it was a step that came "too late to be credible."
Snipers opened fire on the U.N. convoy, hitting one of the vehicles carrying a team on its way to investigate the Aug. 21 incident.
Martin Nesirky, a spokesman for U.N. Secretary-General Ban Ki-moon, said one of the U.N. vehicles was "deliberately shot at multiple times" in the buffer zone between rebel- and government-controlled territory, adding that the team was safe.
Nesirky said the car was "no longer serviceable" after the shooting, forcing the team to return to a government checkpoint to replace the vehicle. U.N. spokesman Farhan Haq said the tires and windshield were hit, but the window was not shattered, and the team plans to go out again Tuesday to do more sampling.
Ban said he had instructed U.N. disarmament chief Angela Kane in Damascus "to register a strong complaint" for the convoy attack with both the Syrian government and opposition representatives.
The Syrian government said its forces provided security for the team until they reached a position controlled by the rebels, where the government claimed the sniper attack occurred. The main Syrian opposition group in exile, the Syrian National Coalition, said members of a pro-government militia known as the Popular Committees fired at the U.N. team to prevent them from going in.
The rebel coalition said the shots occurred near the final checkpoint between rebel and regime-controlled areas, calling it an attempt "to intimidate the U.N. team and prevent it from discovering the truth about Assad's chemical weapons attack against civilians."
Activists said the inspectors eventually arrived in Moadamiyeh, a suburb of Damascus and one of the areas where the alleged chemical attack occurred.
Wassim al-Ahmad, a member of the Moadamiyeh council, said five U.N. investigators spent three hours at a makeshift hospital meeting with doctors and victims still suffering symptoms from the alleged chemical attack, taking blood, hair and tissue samples before returning to Damascus.
"They are late. They came six days late," said al-Ahmad, referring to the time it took the U.N. team to arrive. "All the people have already been buried," he added.