Image copyright AFP Image caption The vessel, the Wise Honest, is accused of having breached international sanctions

North Korea has demanded the immediate return of a cargo ship seized by the US, branding the move an "unlawful robbery".

Last week the US justice department accused Pyongyang of violating international sanctions by using the vessel for coal shipments.

The ship, the Wise Honest, was first impounded in Indonesia in April 2018.

North Korea said its seizure violated the spirit of the 2018 summit agreement between Donald Trump and Kim Jong-un.

"This act is an extension of the US-style calculation of trying to hold us in submission with its 'maximum pressure' and is a total denial of the fundamental spirit of the June 12 DPRK-US joint statement," its foreign ministry said.

It would be the "biggest miscalculation" if the US believes it can control North Korea with force, the statement added.

This is the first time the US has seized a North Korean ship for breaching sanctions and comes amid worsening relations between the two countries.

Image copyright Reuters Image caption North Korea has accused the US of violating the spirit of the agreement the nations' leaders made

The vessel was initially held by Indonesia but the US filed a seizure warrant at a US court. Washington argued that the ship carried illegal coal shipments in violation of UN sanctions and that the cost of maintaining it was illegally paid via unwitting US financial institutions.

North Korea has 60 days to contest the decision by the US court but is not expected to do so.

If the vessel remains in US hands, Washington could sell it.

A second summit between North Korean leader Kim Jong-un and US President Donald Trump ended without agreement in February, with the US insisting North Korea give up its nuclear programme and Pyongyang demanding sanctions relief.

North Korea has carried out two weapons tests in the past weeks in what is widely seen as an attempt to increase pressure on the US.

US officials have stressed that the seizure announcement had nothing to do with the tests.