WASHINGTON — The Obama administration has told the United Nations that Ahmed Abu Khattala, the suspected ringleader of the 2012 attacks on the U.S. Consulate in Benghazi, Libya, was plotting additional attacks on Americans and that the United States conducted the weekend raid that seized him under its right to self-defense.

The United States apparently undertook the raid without the knowledge or permission of the Libyan government. A Libyan Foreign Ministry spokesman condemned it Wednesday as a “regrettable infringement on Libya’s sovereignty,” according to several reports.

But in a one-page letter to the U.N. Security Council dated Tuesday, Samantha Power, the U.S. ambassador, laid out a rationale for why the raid was legal under international law.

A U.S. investigation determined that Abu Khattala was a “key figure” in the 2012 attacks and “that he continued to plan further armed attacks against U.S. persons,” Power wrote. “The measures we have taken to capture Abu Khattala in Libya were therefore necessary to prevent such armed attacks, and were taken in accordance with the United States’ inherent right of self-defense.”

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The U.S. ambassador to Libya, J. Christopher Stevens, and three other Americans died in the 2012 attacks on the consulate and a nearby CIA facility.

International law generally prohibits armed intrusions by one country onto another’s soil without permission from the host government or from the Security Council, but makes an exception for matters of self-defense.

The rationale outlined by Power added a legal policy wrinkle to the raid, which was conducted by U.S. military commandos who were officially acting in support of a small number of FBI agents who accompanied them.

Obama administration officials said Tuesday that the raid was conducted under law-enforcement authority, and they unsealed criminal charges against Abu Khattala from July 2013. The raid did not fall under congressional authorization to use military force against al-Qaida.

Power’s letter suggests that although the Obama administration considered the raid to be a criminal law-enforcement operation for domestic legal-authority purposes, it was simultaneously an act of national self-defense for international-law purposes.

Some Republican lawmakers have criticized the decision to handle Abu Khattala under civilian criminal-justice procedures, saying he should be sent to the prison at Guantánamo Bay, Cuba, for military interrogation and prosecution before a military commission.

While Abu Khattala’s militia, Ansar al-Shariah of Benghazi, shares an ideology with al-Qaida, the United States does not consider it to be affiliated with the terrorist group; the United States is engaged in an armed conflict with al-Qaida, not every Islamist militant in the world.

Jack Goldsmith, a Harvard Law School professor who was a senior Justice Department official in the George W. Bush administration, wrote Wednesday that the critics “don’t have a legal leg to stand on” and that “civilian trial appears to be the only legally available option.”

Abu Khattala is believed to be held on a naval vessel where he is undergoing an FBI-led interrogation. It is not clear whether he has been read a Miranda warning of his right to remain silent and have a defense lawyer; the Obama administration has adopted an expansive interpretation of a “public-safety” exception to that rule for terrorism cases.

It is also not clear whether he has been informed of a right to be presented to a magistrate judge, which federal rules say has to happen “without unnecessary delay,” even for overseas arrests. In several terrorism cases, agents have persuaded suspects to waive that right.