Defense Secretary James Mattis issued a stark warning to the Syrian government on Monday, saying it would be "ill-advised ever again to use chemical weapons."

Mattis released a statement on last Thursday's U.S. missile strikes on a Syrian airfield, which Washington believes the regime of Syrian President Bashar al-Assad used to launch a chemical weapons attack two days earlier.

"The U.S. military strike against Shayrat airfield on April 6 was a measured response to the Syrian government's use of chemical weapons," Mattis said. "The president directed this action to deter future use of chemical weapons and to show the United States will not passively stand by while Assad murders innocent people with chemical weapons, which are prohibited by international law and which were declared destroyed."

The U.S. military fired 59 Tomahawk cruise missiles from the Mediterranean Sea at the airfield in Homs, destroying much of the base and killing at least six people. The strikes, ordered by President Trump, were a response to the Assad's regime use of chemical weapons last Tuesday on a town in northern Syria, where over 80 civilians were killed by the gas attack, including dozens of children.

Mattis' statement provided new information on how damaging the strikes were to the Syrian military.

"The assessment of the Department of Defense is that the strike resulted in the damage or destruction of fuel and ammunition sites, air defense capabilities, and 20 percent of Syria's operational aircraft," the statement said. "The Syrian government has lost the ability to refuel or rearm aircraft at Shayrat airfield and at this point, use of the runway is of idle military interest."

The Pentagon press release came after reports that Syrian military aircraft took off from the airfield the day after the strikes, carrying out further attacks on rebel-held areas.

The Pentagon chief also appeared to imply that further chemical weapons use by the Assad regime would trigger a U.S. military response.

"The Syrian government would be ill-advised ever again to use chemical weapons," Mattis concluded.