DAMASCUS, Syria – U.N. experts collected samples and testimony from Syrian doctors and victims of an alleged chemical weapons attack Monday following a treacherous journey through government and rebel-held territory, where their convoy was hit by snipers.

As U.S. 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 U.S. 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 Shiite Islam.

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.

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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. He said the U.S. has additional information about the attack and will make it public in the days ahead.

“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.

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“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 currently 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” with both the Syrian government and opposition representatives for the convoy attack.

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.

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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 western suburb of Damascus and one of the areas where the alleged chemical attack occurred.

Wassim al-Ahmad, a member of the Moadamiyeh local 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 via Skype, after returning from the hospital where he witnessed the U.N. visit.

The sounds of explosions could be heard in the background. Al-Ahmad said heavy shelling resumed as soon as the U.N. experts left the area following a lull.

In videos uploaded by the Moadamiyeh media office, U.N. inspectors in blue helmets and body armour were seen interviewing hospital patients.

“After the shells landed, I went downstairs and … felt dizzy. I fell down, nauseous. Everything became distorted,” one bearded man was seen telling the U.N. official.

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One video showed a man lying on a stretcher in the presence of U.N. experts and doctors in the room, his legs twitching uncontrollably. In another, a U.N. expert was seen conducting tests on a missile.

The U.S., France, Britain and Israel said a military response against the Syrian regime should be an option. Germany suggested for the first time it may support the use of force if a chemical weapons attack is confirmed.

“The suspected large-scale use of poison gas breaks a taboo even in this Syrian conflict that has been so full of cruelty,” according to Steffen Seibert, a spokesman for Chancellor Angela Merkel.

U.S. Defence Secretary Chuck Hagel, speaking to reporters after meeting with his Indonesian counterpart, said the Obama administration “is considering all different options,” and that “if there is any action taken, it will be in concert with the international community and within the framework of a legal justification.”

French Foreign Minister Laurent Fabius said “all the options are open. The only option that I can’t imagine would be to do nothing.”

Russia said Western nations calling for military action have no proof the Syrian government was behind any chemical attacks.

Syrian activists and opposition leaders have said that between 322 and 1,300 people were killed in the alleged chemical attack.

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Russian Foreign Minister Sergey Lavrov said the countries calling for military action have assumed the role of “both investigators and the U.N. Security Council” in probing the incident.

Lavrov likened the situation in Syria to the period before the U.S.-led invasion of Iraq in 2003. He said “the use of force without a sanction of the U.N. Security Council is a crude violation of the international law.”

Assad told the Russian newspaper Izvestia that accusations his troops used chemicals were “politically motivated.”

“This is nonsense,” Assad was quoted as saying. “First they level the accusations, and only then they start collecting evidence.”

Assad said attacking such an area with chemical weapons would not make sense for the government, because there was no clear front line between regime and rebel forces.

“How can the government use chemical weapons, or any other weapons of mass destruction, in an area where its troops are situated?” he asked.

Commenting on a possible strike by the U.S., Assad said: “They can start a war but they will not know where it will spread or how it will end. Superpowers can launch wars but they cannot win them.”

Asked what the U.S. would face in any intervention, Assad answered: “What it suffered in all its wars from Vietnam until now: Failure.”

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Karam reported from Beirut. Associated Press writers Albert Aji in Damascus, Bassem Mroue in Beirut, Nataliya Vasilyeva in Moscow, Matthew Lee and Julie Pace in Washington, John Heilprin in Geneva, Robert Reid in Berlin, Peter Spielmann in New York contributed to this report.