The United States and its allies waged more than 100 strikes on Syria on Friday in a "one time shot" that the Pentagon said followed conclusive evidence that Syrian President Bashar al-Assad was responsible for a chemical weapons attack using at least chlorine gas.

U.S. Defense Secretary Jim Mattis and Marine General Joseph Dunford said three main chemical weapons facilities were targeted by both missiles from the sea and fired from aircraft, which triggered Syrian air defenses.

The Pentagon could not confirm how many missiles hit their targets.

Mattis and Dunford acknowledged the strike was designed to degrade Syria's chemical weapons capability without killing civilians or the many foreign fighters in Syria's multi-sided civil war, particularly those from Russia.

"We specifically identified these targets to mitigate the risk of Russian forces being involved," Dunford told reporters, adding the U.S. military advised Russia of airspace that would be used in the strike but did not "pre-notify them."

Mattis acknowledged that the United States waged the attacks only with conclusive evidence that chlorine gas was used in the April 7 attack in Syria.

Syria strike Syria air defenses strike back after air strikes by U.S., British and French forces in Damascus, Syria, in this still image obtained from video dated April 14, 2018. Photo: SYRIA TV via Reuters TV

Allegations of Assad's chlorine use are frequent in Syria's conflict, raising questions about whether Washington had lowered the threshold for military action in Syria by now deciding to strike after a chlorine gas attack.

Last year, the United States only waged strikes on Syria after determining that more deadly sarin gas was used and some U.S. media had reported that Washington was confident Assad had also used sarin on April 7.

Mattis, however, suggested the evidence of sarin was so far inconclusive.

"We are very confident that chlorine was used. We are not ruling out sarin right now," Mattis said.

The Pentagon said one of the targets was a scientific research center located in the greater Damascus area, which it described as a Syrian center for the research, development, production and testing of chemical and biological weaponry.

The second target was a chemical weapons storage facility west of the city of Homs.

"We assess that this was the primary location of Syrian sarin and precursor production equipment," Dunford said.

The third target, which was also near Homs, contained both a chemical weapons equipment storage facility and a command post.