Biden sent a letter to the congressional leader explaining the pro-Iran militia airstrike in Syria, after being criticized by some congressmen.

In a letter sent on February 27, US President Joe Biden said the strike against a Syrian base by pro-Iran militia was an act "in accordance with the inherent US right to self-defense, as shown in Article 51.

"Under my direction, the US forces launched on 25 February a deliberate military strike against a facility in eastern Syria, which is used by Iranian-backed militia groups," said Biden.

"These militia groups have previously engaged in attacks against the US and its coalition members in Iraq, including the February 15 attack in Erbil, Iraq that injured one soldier and four military contractors.

Biden asserted that the United States "is always ready to take the necessary and appropriate action in self-defense", even if the national government where the threat is "does not want or cannot prevent militia groups from taking it.

White House press secretary Jen Psaki said on February 26 that the Pentagon gave a brief announcement to congressional leadership before the air strike took place.

The Biden administration said the airstrike campaign was intended to respond to pro-Iranian militia rocket launches against US forces, in accordance with Article 2 of the US Constitution and Article 51 of the UN Charter.

However, some Democrats said that the US Congress did not approve the authorization of the president to use military force in Syria.

"Biden became the seventh consecutive US president to order airstrikes in the Middle East," said Democratic Rep. Ro Khanna.

Some US congressmen over the years have sought to oppose the overinterpretation of the 2001 Authorization to Use Military Forces (AUMF) resolution and replace it with a resolution defining narrower war power.

A spokesman for the US National Security Council said the Biden administration had implemented "a rigorous process that includes review of the legality of the airstrikes deployed".

The US Air Force on February 25 sent two F-15E fighters to throw seven guided bombs at a pro-Iran militia complex in Syria, about 300 meters from the border with Iraq.