US President Joe Biden provided an official explanation for the air strike on Iran-backed fighters in eastern Syria, saying he was using his authority to deter attacks on US and allied personnel in Iraq.
Mr Biden’s letter to House Speaker Nancy Pelosi and Patrick Leahy, deputy leader of the Senate, came after some Democrats criticised the president's first use of military force without informing Congress.
Like a long line of his predecessors, he said he was informing Congress "consistent with" the War Powers Act, without acknowledging that he was required to do so.
Mr Biden said he ordered the strike “to protect and defend our personnel and our partners against these attacks and future such attacks” under his constitutional authority to conduct US foreign policy and as commander-in-chief.
The letter was released by the White House on Saturday.
“Those non-state militia groups were involved in recent attacks against United States and coalition personnel in Iraq.”
The attacks included a February 15 strike in Erbil that wounded a US service member and four American contractors, and killed a Filipino contractor, Mr Biden said.
Tim Kaine, a Democratic senator from Virginia, has long said that presidents need approval from Congress for most military operations.
Mr Kaine said on Friday that Americans should hear the administration’s reasoning for the strikes “and its legal justification for acting without coming to Congress".
Ro Khanna, a representative for California, said there was no justification for a president to order a strike that was not an act of self-defence without congressional authorisation.
In his letter, Mr Biden said he was acting under the country's inherent right to self-defence as reflected in the UN charter.
White House press secretary Jen Psaki said the Defence Department briefed congressional leaders before the strikes.
James Inhofe, a senator for Oklahoma and the top Republican on the Senate armed services committee, said on Friday that the “strikes were the correct, proportionate response to protect American lives".
"I look forward to more information on the administration’s response to Iran’s aggression," Mr Inhofe said.
Other Republicans echoed his approval.
The US destroyed nine bases and damaged two in the attack, Pentagon spokesman John Kirby said on Friday.
Accounts of how many fighters were killed varied widely, with Iran-backed Iraqi militia Kataib Hezbollah claiming it suffered one casualty.
At least 22 Iraqi militants allied with Iran were killed and three ammunition lorries were destroyed in the attack, the Syrian Observatory for Human Rights said.
Neither report has yet been independently verified, although the Pentagon said the attack was proportionate, suggesting casualties would be closer to the lower figure.
After a decade of civil war, Syria’s military is not in a strong position to respond directly to a US attack.
Syria was twice attacked by the US military during former president Donald Trump’s term, both over Syrian President Bashar Al Assad’s use of chemical weapons in the conflict.