File Photo: U.S. President Donald Trump delivers a speech at the White House, the United States, on April 12, 2018. (Xinhua/Yang Chenglin)

WASHINGTON, April 19 (Xinhua) -- The White House on Thursday denied that Secretary of Defense James Mattis had advised U.S. President Donald Trump against striking Syria before seeking Congress sanction.

"Reports that Secretary Mattis urged the President to seek congressional approval before last week's strikes in Syria are categorically false," a statement from White House Press Secretary Sarah Sanders said.

"As Secretary Mattis explained to Congress in yesterday's all-member briefs, the President appropriately ordered the strikes under his constitutional authorities," the statement said.

The response came after U.S. media cited military and administration sources as saying that Mattis had urged Trump to seek Congressional approval before punishing Syria for alleged use of chemical weapons. Trump, who was said to be eager to back up his fiery tweets against the Syrian government, reportedly then overruled Mattis.

The United States, in coordination with the United Kingdom and France, fired a barrage of missiles against three targets in Syria that it alleged contained, manufactured or researched chemical weapons.

The attack was carried out without sanction from the United Nations Security Council in accordance with the UN charter, nor was it based on any findings from the Organization for the Prohibition of Chemical Weapons, a UN watchdog tasked to investigate alleged uses of chemical weapons in Syria.

The United States, with its allies, said they have obtained evidence to support their claim, but has not yet released any of the evidence.

The attack came one year after U.S. forces took out a Syrian government military airfield, which it said supported chemical attacks.