North Korea on Tuesday called the seizure of its cargo ship by the United States "an act of robbery" and demanded immediate release of the vessel impounded last week for evading international sanctions.

In a statement by the state-run KCNA, a spokesperson for North Korean ministry of foreign affairs, condemned the seizure of Wise Honest ship and said it was in violation of the spirit of last year's Singapore agreement between US President Donald Trump and North Korea leader Kim Jong-un.

The spokesperson said by impounding the North Korea-flagged cargo vessel, the US had committed an "illegal and atrocious act of robbery" using the sanctions as an excuse.

"The US behavior is the extension of its calculation to make us succumb to the so-called maximum pressure," the spokesperson said.

The ministry said Washington "must return our vessel without any delay", and warned that the Trump administration should mull over "what repercussions its gangster-like act will entail".

The May 9 seizure of Wise Honest, one of Pyongyang's largest bulk carriers, came after North Korea test-fired two short-range ballistic missiles on last Wednesday, its second such launch in five days.

The weapons test was conducted after a pause of 17 months to build up pressure on Washington amid stalled denuclearization talks since the Feb 28 Hanoi summit between Trump and Kim Jong-un.

Pyongyang had requested for a gradual denuclearization and ease of sanctions, considered to be unacceptable by the White House. The Trump administration is in favor of immediate North Korean disarmament.

The 17,061-ton, single-hull bulk carrier ship was first seized by Indonesian authorities near its coast in April 2018. The US announced its seizure last week, the first time Washington had impounded a North Korean carrier for alleged violation of international sanctions.

"The Wise Honest was used to illicitly ship coal from North Korea and to deliver heavy machinery to the (North Korea). Payments for maintenance, equipment, and improvements of the Wise Honest were made in US dollars through unwitting US banks. This conduct violates longstanding US law and United Nations Security Council resolutions," the Department of Justice said in a May 9 statement.

The statement added that "the time when the US could make the world go around at its own will is past" and said that North Korea will continue to "sharply watch the future movement of the US".