NEW YORK, June 29 (Reuters) - The United States told the U.N. Security Council on Tuesday that it targeted Iran-backed militia in Syria and Iraq with airstrikes to deter the militants and Tehran from conducting or supporting further attacks on U.S. personnel or facilities. Under Article 51 of the United Nations Charter, the 15-member Security Council must immediately be informed of any action that states take in self-defense against armed attack. U.S. Ambassador to the United Nations, Linda Thomas-Greenfield, said the airstrikes hit facilities used by militia blamed for an escalating series of drone and rocket attacks against U.S. personnel and facilities in Iraq. "This military response was taken after non-military options proved inadequate to address the threat, with the aim of deescalating the situation and preventing further attacks," she wrote in a letter, seen by Reuters. President Joe Biden wrote a similar letter to Congress on Tuesday. "The United States stands ready to take further action, as necessary and appropriate, to address further threats or attacks," he said. U.S. troops came under rocket fire in Syria on Monday in apparent retaliation for weekend U.S. air strikes. About 34 rockets were fired in the attack, but there were no injuries, a U.S. military official said on Tuesday. read more While the Iraqi and U.S. militaries coordinate closely in a separate battle in Iraq against remnants of the Sunni extremist group Islamic State, Iraq"s government and military condemned the U.S. airstrikes against the Iran-back militia. The United States began airstrikes against Islamic State in Syria in 2014, telling the United Nations it acted because Syrian President Bashar al-Assad"s government had failed to wipe out safe havens used by the militants to launch attacks on Iraq. Iraq told the U.N. Security Council at the time that it requested U.S. help because a safe haven for the militants in Syria had made its border "impossible to defend."