WASHINGTON — President Trump said Thursday night that the United States had carried out a missile strike in Syria in response to the Syrian government’s chemical weapons attack this week, which killed more than 80 civilians.

“Tonight, I ordered a targeted military strike on the air base in Syria from where the chemical attack was launched,” Mr. Trump said in remarks at his Mar-a-Lago estate in Florida. “It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.”

Mr. Trump — who was accompanied by senior advisers, including Stephen K. Bannon, his chief strategist; Reince Priebus, his chief of staff; his daughter Ivanka Trump; and others — said his decision had been prompted in part by what he called the failures by the world community to respond effectively to the Syrian civil war.