President Joe Biden arrives at Ellington Field Joint Reserve Base in Houston, Texas, U.S., February 26, 2021.Jonathan Ernst | ReutersPresident Joe Biden on Saturday told Congress that the airstrikes he ordered in Syria this week were consistent with the U.S. right to self-defense, as members of his own party called for more transparency about why military action was taken without congressional authorization. “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 wrote in a letter to House Speaker Nancy Pelosi and President pro tempore of the Senate Patrick Leahy. Biden ordered airstrikes Thursday against facilities in eastern Syria that the Pentagon says are used by Iranian-backed militia. The Defense Department said multiple facilities were destroyed at a border control point and there were casualties, but did not provide any additional information. Those strikes came in response to a Feb. 15 attack in which rockets hit Erbil International Airport, which hosts a coalition military base, in northern Iraq. The attack killed a civilian contractor with the U.S.-led military coalition and injured several others, including an American service member. “I directed this military action to protect and defend our personnel and our partners against these attacks and future such attacks,” Biden wrote in his letter Saturday.The letter comes as some Senate Democrats have pushed back against Biden over the strikes, calling on him to provide a briefing why military action was taken without congressional approval. Under the War Powers Resolution, presidents are required to inform Congress within 48 hours after taking military action. In the letter, Biden cited his constitutional authority as commander in chief. “I directed this military action consistent with my responsibility to protect United States citizens both at home and abroad and in furtherance of United States national security and foreign policy interests, pursuant to my constitutional authority to conduct United States foreign relations and as Commander in Chief and Chief Executive,” Biden wrote.The Pentagon briefed leaders in Congress before the military strikes, according to a National Security Council spokesman. House Speaker Nancy Pelosi’s staff were also notified before the strike, according to a Democratic aide.Iran condemned the U.S. air strikes on Saturday and denied responsibility for the rocket attacks on U.S. targets. Iranian Foreign Minister Mohammad Javad Zarif said the U.S. strikes were “illegal and a violation of Syria’s sovereignty,” according to reports from Iran’s state media.— CNBC’s Christian Nunley and Reuters contributed to this report.