North Korea is denouncing the United States as "a gangster country" for recently seizing one of its cargo ships and is calling on 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 the ship.

The U.S. says it seized the ship for carrying coal in violation of U.N. sanctions. It was a first-of-its kind enforcement action at a tense moment in U.S-North Korean relations.

Kim said concern is growing over the possible impact of the "heinous" U.S. act on the situation in the Korean peninsula but didn't say what "urgent measures" Guterres should take.