PHOTO: pool
Now playing
02:29
Pentagon: No doubt Syria behind gas attack
PHOTO: pool
Now playing
01:00
Mattis: No doubt Syria behind gas attack
2 spicer whpb 3.29
PHOTO: CNN
2 spicer whpb 3.29
Now playing
01:20
Spicer: If you gas a baby, expect a response
MEDITERRANEAN SEA - APRIL 7:  In this handout provided by the U.S. Navy,The guided-missile destroyer USS Porter fires a Tomahawk land attack missile on April 7, 2017 in the Mediterranean Sea. The USS Porter was one of two destroyers that fired a total of 59 cruise missiles at a Syrian military airfield in retaliation for a chemical attack that killed scores of civilians this week. The attack was the first direct U.S. assault on Syria and the government of President Bashar al-Assad in the six-year war there.  (Photo by Ford Williams/U.S. Navy via Getty Images)
PHOTO: U.S. Navy/Getty Images
MEDITERRANEAN SEA - APRIL 7: In this handout provided by the U.S. Navy,The guided-missile destroyer USS Porter fires a Tomahawk land attack missile on April 7, 2017 in the Mediterranean Sea. The USS Porter was one of two destroyers that fired a total of 59 cruise missiles at a Syrian military airfield in retaliation for a chemical attack that killed scores of civilians this week. The attack was the first direct U.S. assault on Syria and the government of President Bashar al-Assad in the six-year war there. (Photo by Ford Williams/U.S. Navy via Getty Images)
Now playing
04:10
Did WH just draw a new red line on Syria?
White House press secretary Sean Spicer speaks during the daily press briefing, Thursday, March 30, 2017, at the White House in Washington.
PHOTO: Evan Vucci/AP
White House press secretary Sean Spicer speaks during the daily press briefing, Thursday, March 30, 2017, at the White House in Washington.
Now playing
01:44
Spicer: Possibility of more action in Syria
3067918 04/07/2017 The United States launched a military strike on a Syrian airbase Thursday. A plane in a shelter. Mikhail Voskresenskiy/Sputnik via AP
PHOTO: Mikhail Voskresenskiy/Sputnik via AP
3067918 04/07/2017 The United States launched a military strike on a Syrian airbase Thursday. A plane in a shelter. Mikhail Voskresenskiy/Sputnik via AP
Now playing
01:22
Syrian airbase reopens after US strikes
Now playing
03:57
Zakaria: Did Syria strike end 'America First'?
Now playing
02:23
Skepticism lacking in Syria strike coverage?
Now playing
01:59
More details emerge of Syria military strike
syria strike animation orig_00001406.jpg
syria strike animation orig_00001406.jpg
Now playing
00:45
US strike on Syria: An animated map
Now playing
01:04
Aftermath of US strike on Syria airbase
Syrians look down at a poster of Syrian President Bashar al-Assad (L) and his late father and predecessor Hafez al-Assad in Damascus, as they watch the tourist train pass following the re-opening ceremony of the rail route between two neighbourhoods in the Syrian capital, Raboeh and Dumar, on May 1, 2015. AFP PHOTO / LOUAI BESHARA        (Photo credit should read LOUAI BESHARA/AFP/Getty Images)
PHOTO: LOUAI BESHARA/AFP/AFP/Getty Images
Syrians look down at a poster of Syrian President Bashar al-Assad (L) and his late father and predecessor Hafez al-Assad in Damascus, as they watch the tourist train pass following the re-opening ceremony of the rail route between two neighbourhoods in the Syrian capital, Raboeh and Dumar, on May 1, 2015. AFP PHOTO / LOUAI BESHARA (Photo credit should read LOUAI BESHARA/AFP/Getty Images)
Now playing
01:17
What is Bashar al-Assad's goal?
Senator Marco Rubio on New Day
Senator Marco Rubio on New Day
Now playing
01:34
Rubio: Syria strike was legal and right move
Trump Syria strike refugees newday_00000000.jpg
Trump Syria strike refugees newday_00000000.jpg
Now playing
02:44
Will Trump now accept Syrian refugees?
MEDITERRANEAN SEA - APRIL 7:  In this handout provided by the U.S. Navy,The guided-missile destroyer USS Porter fires a Tomahawk land attack missile on April 7, 2017 in the Mediterranean Sea. The USS Porter was one of two destroyers that fired a total of 59 cruise missiles at a Syrian military airfield in retaliation for a chemical attack that killed scores of civilians this week. The attack was the first direct U.S. assault on Syria and the government of President Bashar al-Assad in the six-year war there.  (Photo by Ford Williams/U.S. Navy via Getty Images)
PHOTO: U.S. Navy/Getty Images
MEDITERRANEAN SEA - APRIL 7: In this handout provided by the U.S. Navy,The guided-missile destroyer USS Porter fires a Tomahawk land attack missile on April 7, 2017 in the Mediterranean Sea. The USS Porter was one of two destroyers that fired a total of 59 cruise missiles at a Syrian military airfield in retaliation for a chemical attack that killed scores of civilians this week. The attack was the first direct U.S. assault on Syria and the government of President Bashar al-Assad in the six-year war there. (Photo by Ford Williams/U.S. Navy via Getty Images)
Now playing
01:25
Oil prices jump after U.S. strikes Syria

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.

“Our priority remains the defeat of ISIS,” Mattis said, labelling the terror group “a clear and present danger.”

“The purpose of this attack was singularly against chemical weapons use,” Mattis told reporters, saying preventing the use of chemical weapons represented “a vital national interest” of the United States in order to ensure that such weapons of war do not become “commonplace.”

“The reason for the strike was that alone,” he said. “It was not a harbinger of some change in our military campaign.”

Mattis cited the damage to Assad’s air force from the missile strike as one reason why the Syrian leader might regret his decision to use sarin gas as a weapon.

A US defense official told CNN that nearly two dozen Syrian aircraft were destroyed in the missile attack, which Pentagon officials say equates to about 20% of the regime’s operating air force.

Mattis also offered a stark warning to the Syrian leader.

“I recommend Assad be rather cautious about violating international law with chemical weapons,” he said. “I suppose that could be considered a red line.”

“The Syrian regime should think long and hard before it again acts so recklessly in violation of international law against the use of chemical weapons,.” he added

Some ambiguity remains, however, on whether the most commonly used chemical weapon in the Syrian civil war, chlorine-filled barrel bomb dropped from aircraft, would warrant US military action.

International monitoring groups, including the Organization for Prohibition of Chemical Weapons and the United Nations Joint Investigative Mechanism, have accused Assad’s air force of using such weapons multiple times

Like sarin, weaponized chlorine is prohibited under the convention outlawing chemical weapons.

“It is not about whether it’s delivered with an artillery shell, or it’s delivered by a helicopter with a barrel bomb, or a fighter aircraft with a bomb,” Mattis said. “It’s about chemical weapons.”