WASHINGTON, April 11 (Xinhua) -- The United States is still focused on defeating the Islamic State group in Syria, U.S. Defense Secretary James Mattis said Tuesday, days after U.S. missile strikes on a Syria airbase.

"Our military policy in Syria has not changed. Our priority remains the defeat of ISIS (IS). ISIS represents a clear and present danger, an immediate threat to Europe and ultimately, a threat to the U.S. homeland," Mattis said at his first Pentagon news conference as defense chief.

The United States on Thursday launched 59 Tomahawk missiles at a Syrian airbase, claiming its military action was meant to neutralize Syria's chemical weapon arsenal, two days after reports of a chemical weapons attack emerged from the battleground in Syria.

While the West said the Syrian government should be to blame, Damascus denied that.

The Russian government has condemned the missile strikes by the United States, calling Washington's move "aggression against a sovereign state in violation of international law."

Mattis said the military action demonstrates the United States would not passively stand by while Syrian President Bashar al-Assad "blithely ignores international law and employs chemical weapons he had declared destroyed."

Mattis also warned that if Syria were to use chemical weapons again, it would "pay a very, very stiff price."

"The Syrian regime should think long and hard before it again acts so recklessly in violations to international law against the use of chemical weapon," the Pentagon chief said.