The parents of a US student who died after being imprisoned in North Korea have filed a claim against one of the Asian country's ships.

Otto Warmbier's family are trying to collect some of a $501m judgement awarded to them, AP reported.

They are claiming a right to the unnamed vessel, which the US seized in May because it was carrying coal in violation of UN sanctions.

They filed papers in New York federal court last week.

Warmbier had travelled to North Korea on a guided tour.

His parents allege he was tortured after being convicted of trying to steal a propaganda poster.

Warmbier, 22, suffered severe brain damage, but doctors were unable to determine the cause.

He died days after being returned to the US in a vegetative state in 2017.