North Korea on Tuesday denounced the impounding of one of its cargo ships as "unlawful robbery" and demanded the United States return the vessel without delay.

In a statement from the North Korean foreign ministry, it said: "The United States carried out an illegal act of robbery by seizing our cargo ship citing UN Security Council sanctions resolutions."

Relations between the two nations had initially thawed following a summit agreement between Trump and Kim Jong Un on June 12 last year. Kim and Trump had seemingly agreed to a nuclear-free Korean Peninsula and improved bilateral ties. However, this latest move was "a total denial of the fundamental spirit" of that meeting according to the statement.

In a thinly veiled threat, it added: "The US must realize the consequence of its gangster-like actions... and must return our ship without delay."

Despite last year's accord, a second Trump-Kim Jong Un meeting in February 2019 proved to be less fruitful as talks broke down without a deal.

North Korea was reacting to last week's announcement that the US Justice Department had seized a vessel accusing it of illicit coal shipments. The ship, known as the "Wise Honest", had initially been detained by Indonesian officials in April 2018 but has since been impounded by the US and taken to American Samoa.

North Korea is banned from exporting coal under United Nations sanctions in 2017. Experts believe coal and other mineral exports help finance its weapons industry.

Weapons testing have resumed on the peninsula with two recent rounds of test firings of short-range missiles in a week.

Silk/rc (Reuters, AP, AFP)