Story highlights Mattis said there was 'no doubt' the government of Bashar al-Assad was responsible for the chemical attack

Mattis sought to strike a clear message about US military objectives and priorities in Syria

(CNN) Secretary of Defense James Mattis said Tuesday that America's top priority in Syria remained fighting the Islamic State of Syria and Iraq, but he made it clear the use of chemical weapons would not be tolerated and could warrant additional military actions like last week's missile strike against a Syrian air base.

"If they use chemical weapons, they are going to pay a very, very stiff price," Mattis told reporters during his first Pentagon briefing as secretary.

Mattis said there was "no doubt" the government of Bashar al-Assad was responsible for the chemical attack in Syria last week that killed scores of civilians, including children, but he did not say whether he believed Syria's key ally, Russia, was complicit.

"There's no doubt the Syrian regime is responsible for the decision to attack," Mattis said, adding, "The Assad regime planned it, orchestrated it and executed it, and beyond that we can't say right now."

Mattis sought to strike a clear message about US military objectives and priorities in Syria to eliminate any confusion about the scale of Thursday's intervention in the wake of mixed messages from the administration regarding regime change in Damascus. The strike against al-Shayrat airfield was tied specifically to Damascus' decision to use chemical weapons and did not indicate a wider military campaign aimed at ousting Assad, he said.

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