Washington announced last week it had taken possession of the North Korean-registered bulk carrier M/V Wise Honest -- a year after it was detained in Indonesia -- citing sanctions-violating activities.
The seizure came amid heightened tensions after Pyongyang conducted weapons drills involving short-range missiles in recent weeks, and with nuclear talks deadlocked since the collapse of the Hanoi summit between US President Donald Trump and North Korean leader Kim Jong Un earlier this year.
In a letter sent Friday to Secretary-General Antonio Guterres, Kim Song -- Pyongyang's permanent representative at the UN -- said the incident was "an unlawful and outrageous act", according to North Korea's state news agency KCNA.
"This act of dispossession has clearly indicated that the United States is indeed a gangster country that does not care at all about international laws," the letter said.
The North Korean representative asked Guterres to "take urgent measures as a way of contributing to the stability of the Korean peninsula and proving the impartiality of the UN".
Earlier this week, Pyongyang had slammed the seizure as an "outright denial" of the spirit of a statement signed by Kim and Trump at their first summit in Singapore last year.
North Korea is sanctioned under multiple UN Security Council resolutions for its nuclear and missile programmes, and lifting of some of the measures was a key demand from Pyongyang at the second Trump-Kim summit in February that ultimately broke down without a deal.