The U.S. military on Thursday launched a targeted missile strike at a Syrian military airfield in its first direct assault on the army of Syrian President Bashar al-Assad since Syrian crisis began six years ago.

A total of 59 Tomahawk Land Attack missiles were launched from the destroyers USS Porter and USS Ross in the Eastern Mediterranean Sea at about 8:40 p.m. EDT (4:40 a.m.on Friday in Syria), and Syrian aircraft, hardened aircraft shelters, fuel points, air defense systems, and radars were targeted, according to a Pentagon statement.

The strike was ordered by U.S. President Donald Trump, said the statement, adding that the assault was in response to the alleged Syrian government's chemical weapons attack on Tuesday in the rebel-held town of Khan Sheikhoun, which killed over 70 people and wounded scores of others, most of whom were civilians.

Speaking to reporters at his Mar-a-Lago resort in Florida, Trump said the strike was in the nation's "vital national interest."

"It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons," said Trump. "The refugee crisis continues to deepen and the region continues to destabilize."

Syrian Foreign Minister Walid al-Moallem said early on Thursday that the Syrian airstrike on a rebel-held town in Idlib Province struck a rebel depot containing chemical materials, denying that the air force fired toxic gas during the attack.

In a previous press conference held to comment on the global accusation of Syria's allegedly firing toxic gas on Khan Sheikhoun on Tuesday, al-Moallem said the news reports are "lies."

The minister also insisted that "the Syrian army forces haven't and will not use chemical weapons," and that the forces are no longer in possession of such weapons.

After the U.S. strike, Syrian state TV called the assault an "aggression".

Despite the Syrian government's denial of using chemical weapons, U.S. Secretary of State Rex Tillerson said Thursday night that the United States had a "high level of confidence" that the Syrian government used the chemical weapons and accused Russia of failing in its responsibility to deliver on a 2013 deal it helped broker to destroy Syria's chemical weapons arsenal.

"So either Russia has been complicit or either Russia has been simply incompetent in its ability to deliver on its end of that agreement," Tillerson told reporters.

Tillerson added that no contacts were made with Moscow on the U.S. strike, though U.S.-Russian military de-confliction agreements, reached in 2015 to avoid mid-air incidents in the skies over Syria in the fight against the Islamic State (IS), were followed.

The U.S. strike against Syrian army marked a major departure from the policy of the previous U.S. government under President Barack Obama, who ordered strikes against the IS rather than Syrian government forces.

However, the move did not necessarily represent a change in U.S. policy or posture in Syria.

"There has been no change in that status," said Tillerson. "It does demonstrate that President Trump is willing to act when governments and actors cross the line."

The Pentagon also said in its statement that the strike was intended to "deter the Syrian government from using chemical weapons again."

Adam Schiff, the ranking member of the House Intelligence Committee, told MSNBC on Thursday night that there were "no current plans for additional strikes" after receiving a briefing by U.S. Director of National Intelligence Dan Coats.

The strike also came one week after U.S. Ambassador to UN Nikki Haley said the U.S. diplomatic policy on Syria crisis for now was not focused on "getting Assad out."

However, Trump indicated on Wednesday that after the alleged chemical weapons attack by the Syrian government, his thinking about Assad was transformed.

"I will tell you that attack on children yesterday had a big impact on me - big impact," Trump said during his joint press conference with King Abdullah of Jordan in the White House Rose Garden. "My attitude toward Syria and Assad has changed very much ... You're now talking about a whole different level."