DAMASCUS — UN inspectors on Monday collected samples and testimony from Syrian doctors and victims of an alleged chemical weapons attack after a treacherous journey through government and rebel-held territory, where their convoy was hit by snipers.
As US officials said there was very little doubt that Syria used chemical weapons and Western powers stepped up calls for swift military action, President Bashar Assad’s government vowed to defend itself against any international attack, warning that such an intervention would ignite turmoil across the region.
It also would bring the United States closer to a conflict that has killed more than 100,000 people since Assad cracked down on Arab Spring-inspired protesters in March 2011.
Syria’s civil war has been increasingly defined by sectarian killings between the Sunni-led rebellion and Assad’s regime, dominated by Alawites, an offshoot of Shi’ite Islam.
It would essentially pit the United States 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.
In addition to the United States, Britain, France, Germany, Israel, and Turkey said a military response against the regime should be an option.
Syria’s deputy foreign minister, Faysal Mikdad, said that 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 was unclear what US action would mean for relations with Russia, which warned Monday against the use of force not sanctioned by the United Nations Security Council, calling it ‘‘a crude violation of international law.’’
Support for some sort of international military response was expected to grow if it is confirmed that Assad’s regime was responsible for an 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.
Secretary of State John Kerry said chemical weapons were used in Syria, and he accused Assad’s regime of destroying evidence. He said the United States has additional information about the attack and will make it public.
Assad has denied launching a chemical attack, blaming the rebels instead, and has authorized a UN team of experts currently in Syria to investigate, although the United States said it was a step that came ‘‘too late to be credible.’’
Snipers opened fire on the UN convoy, hitting one of the vehicles. Martin Nesirky, a spokesman for UN Secretary General Ban Ki-moon, said one of the vehicles was ‘‘deliberately shot at multiple times’’ in the buffer zone between rebel- and government-controlled territory, adding that the team was safe. UN spokesman Farhan Haq said the team plans to go out again Tuesday to do more sampling.
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 UN team to prevent it from going in.
Activists said the inspectors eventually arrived in Moadamiyeh, a western suburb of Damascus and one of the areas where the chemical attack allegedly occurred.
Wassim al-Ahmad, a member of the Moadamiyeh local council, said UN investigators spent three hours at a makeshift hospital meeting with doctors and victims, taking blood, hair, and tissue samples before returning to Damascus.
‘‘They are late. They came six days late,’’ said Ahmad, referring to the time it took the UN team to arrive. ‘‘All the people have already been buried,’’ he added via Skype.
Defense Secretary Chuck Hagel said the Obama administration ‘‘is considering all different options,’ and ‘‘if there is any action taken, it will be in concert with the international community and within the framework of a legal justification.’’
Germany suggested for the first time Monday that it may support the use of force. ‘‘The suspected large-scale use of poison gas breaks a taboo even in this Syrian conflict that has been so full of cruelty,’’ said Steffen Seibert, a spokesman for Chancellor Angela Merkel.
France’s foreign minister, Laurent Fabius, said ‘‘all the options are open. The only option that I can’t imagine would be to do nothing.’’