President Joe Biden demanded Congress to support his first use of military force in Syria last week, stressing that the decision was consistent with the US right to self-defense.
The US military launched an airstrike on facilities in eastern Syria used by Iran-backed militias Thursday, the Pentagon said.
Some Democrats have criticized the Biden administration for the strike and demanded a briefing after saying that military action without congressional approval is not constitutional.
In his letter to House Speaker Nancy Pelosi and Patrick Leahy, president pro tempore of the Senate, Biden said non-state militia groups were involved in recent attacks against US and Coalition personnel in Iraq.
“I directed this military action to protect and defend our personnel and our partners against these attacks and future such attacks,” he wrote in the letter.
The orders came under his constitutional authority to conduct US foreign policy and as commander-in-chief.
“The United States took this action pursuant to the United States' inherent right of self-defense as reflected in Article 51 of the United Nations Charter," Biden added.
Biden said the US strikes were in response to the Feb. 15 "attack in Erbil, Iraq, which wounded one United States service member, wounded four United States contractors, including one critically, and killed one Filipino contractor. "
He wrote that the United States always stands ready to take necessary and proportionate action in self-defense, including when, as is the case here, the government of the state where the threat is located is unwilling or unable to prevent the use of its territory by non-state militia groups responsible for such attacks.
The US President concluded by saying that he was "providing this report as part of my efforts to keep the Congress fully informed, consistent with the War Powers Resolution."
Iran on Saturday condemned the US strike in Syria, and denied responsibility for attacks on US targets.
Iranian Foreign Minister Mohammad Javad Zarif called the strike “illegal and a violation of Syria’s sovereignty.”