North Korea denounced the United States as “a gangster country” for seizing one of its cargo ships and urged U.N. Secretary-General Antonio Guterres to take “urgent measures” to contribute to stabilizing the Korean peninsula.

North Korea’s U.N. ambassador, Kim Song, said in a letter to Guterres obtained Monday by The Associated Press that the Trump administration committed “an unlawful and outrageous act” by taking over the ship Wise Honest and moving it to American Samoa. He called the seizure a violation of international law and the U.N. Charter.

Kim called a news conference at U.N. headquarters on Tuesday to further criticize the ship’s seizure.

The U.S. said on May 9 that it seized the ship because it was carrying coal in violation of U.N. sanctions, a first-of-its kind enforcement action that came amid a tense moment in relations with North Korea.

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The Wise Honest, North Korea’s second largest cargo ship, was first detained in April 2018 by Indonesia while transporting a large amount of coal.

U.S. Justice Department lawyers laid out the case for confiscating the ship in a complaint filed in New York, arguing that payments for maintenance and operation of the vessel were channeled through unwitting U.S. financial institutions in violation of American law. North Korea’s coal trade itself is also believed to fund the isolated country’s nuclear weapons and ballistic missile programs which the U.S. is seeking to eliminate.

Last week, the North’s official Korean Central News Agency carried a statement by an unidentified foreign ministry spokesman who accused the United States of betraying the spirit of a summit agreement last June between North Korean leader Kim Jong Un and President Donald Trump.

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Kim and Trump agreed then to a vague statement calling for a nuclear-free Korean Peninsula and improved bilateral ties, but a second meeting between the leaders collapsed in February over mismatched demands in sanctions relief and disarmament.

The U.S. announcement of the seizure of the Wise Honest came hours after North Korea fired two suspected short-range missiles toward the sea, the second weapons launch in five days and a possible signal that stalled talks over its nuclear weapons program are in trouble.

The North Korean ambassador said in the letter to Guterres that “since international concern is growing more than ever over the likely impact to be brought to the situation of the Korean peninsula by the heinous act of the United States” in seizing the ship, the secretary-general should take “urgent measures” to stabilize the Korean peninsula. The letter did not specify what those measures should be.