President Trump on Saturday delivered his justification to Congress for ordering a missile strike on Syria this week, saying in a letter to congressional leaders that the U.S. was prepared to take further military action if necessary.

"I acted in the vital national security and foreign policy interests of the United States, pursuant to my constitutional authority to conduct foreign relations and as Commander in Chief and Chief Executive," Trump wrote.

"The United States will take additional action, as necessary and appropriate, to further its important national interests," he added.

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The letter was addressed to House Speaker Paul Ryan Paul Davis RyanAt indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates Peterson faces fight of his career in deep-red Minnesota district MORE (R-Wis.) and Sen. Orrin Hatch Orrin Grant HatchBottom line Bottom line Senate GOP divided over whether they'd fill Supreme Court vacancy MORE (R-Utah), the Senate president pro tempore.

Under the War Powers Resolution, the president is required to submit an explanation for the use of force within 48 hours after military action is taken. The deadline for Trump to do so would be Saturday night.

Trump's letter echoed his comments delivered roughly an hour after the strikes on Thursday night, when he characterized the strikes as in the "vital national security interest" of the U.S.

“It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons,” Trump said at his Mar-a-Lago resort in Florida, where he was hosting Chinese President Xi Jinping.

A number of world leaders rallied around the U.S. strikes on Friday, applauding the action as a necessary and proportional response to Syrian President Bashar Assad’s alleged use of chemical weapons in the country’s Idlib Province.

The Syrian government and Russia, a longtime backer of Assad and one of the regime’s fiercest military supporters, condemned the strike. A spokesman for Russian President Vladimir Putin on Friday called the strike an act of “aggression against a sovereign state” and accused the U.S. of violating international law.

U.S. lawmakers expressed general support for the attack, though many called for the president to seek congressional approval before conducting any further military operations against Syria.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE (R-Ky.), however, said that the president shouldn’t have to seek further approval for military action.

He argued that an Authorization for Use of Military Force (AUMF) approved by Congress in the days after the Sept. 11, 2001 attacks, and another in 2002 that authorized the Iraq War, justified recent military action in the region.

"We passed one back in 2001 and 2002, I believe, and the previous president thought that it authorized what we were doing in that part of the world, and I expect this president thinks the same," he told conservative radio host Hugh Hewitt on Friday.

Trump’s assertion that the U.S. was prepared to carry out further attacks on Syria if necessary echoed similar comments made by U.S. Ambassador to the United Nations Nikki Haley on Friday, who delivered a rebuttal to Russia’s condemnations and defended the president’s action.

"The United States took a very measured step last night," Haley said. "We are prepared to do more, but we hope that will not be necessary."

Updated: 3:51 p.m.