The United States told the United Nations that the killing of Iranian commander Qassem Soleimani was self-defense and vowed to take additional action "as necessary" in the Middle East to protect US personnel and interests. In a letter to the UN Security Council, US Ambassador Kelly Craft said the United States also stands "ready to engage without preconditions in serious negotiations with Iran, with the goal of preventing further endangerment of international peace and security or escalation by the Iranian regime."
What
  1. The killing of Soleimani in Baghdad was justified under Article 51 of the UN Charter, wrote Craft in the letter seen, adding "the United States is prepared to take additional actions in the region as necessary to continue to protect US personnel and interests.
  2. Under Article 51, countries are required to "immediately report" to the 15-member Security Council any measures taken in exercising the right of self-defense. 
  3. The United States used Article 51 to justify taking action in Syria against Islamic State militants in 2014.
Article 51
  1. Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security
  2. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.