The Syrian army said a U.S. missile attack on one of its airbases killed six people and caused extensive damage, adding it would respond by continuing its campaign to "crush terrorism" and restore peace and security to all of Syria.

A statement from the army command described the attack on Friday as an act of "blatant aggression", saying it had made the United States "a partner" of Islamic State, the ex-Nusra Front and other "terrorist organisations".

The United States on Friday fired dozens of cruise missiles at an airbase from which it said a deadly chemical weapons attack was launched this week, in an escalation of the U.S. military role in Syria that directly raised tension with Russia.

U.S. President Donald Trump said he had ordered the strikes in America's "national security interest".

The Syrian army said the attack "undermines the operation of combatting terrorism that the Syrian Arab Army is carrying out".

"The United States has tried to justify this aggression under the pretext of the Syrian Arab Army using chemical weapons in Khan Sheikhoun, without knowing the reality of what happened," it added.

Washington has blamed Assad's forces for the attack that killed scores of people in rebel-held Khan Sheikhoun on Tuesday. The Syrian government has strongly denied responsibility and blamed the deaths on leaks from a rebel chemical arms store it says was hit by a Syrian air strike. 

Rebels say there were no military positions in the targeted area in Khan Sheikhoun and that they have no chemical weapons.

The army said Friday's U.S. attack "sends the wrong messages to the terrorist organizations and makes them go further in using chemical weapons in the future whenever they face big losses on the battlefield".

The Syrian government describes all armed groups opposed to it as terrorists.