Just hours after North Korea launched another round of short-range missiles, its second launch in under a week, the Washington Post is reporting that US authorities have seized a North Korean ship used to sell coal in violation of international sanctions.

The FBI and federal prosecutors have confirmed the ship, "the Wise Honest", is approaching US territorial waters. The US Marshals and the Coast Guard are helping to coordinate its journey.

"This sanctions-busting ship is now out of service," Assistant Attorney General John Demers said in announcing the seizure.

The ship is one of North Korea's largest, weighing in at 17,601 tonnes and illicitly used to ship coal. The ship was first stopped last year by Indonesian authorities on suspicion of violating sanctions.

The seizure marks an escalation of the American government's efforts to stop Pyongyang's rampant sanctions violations. It also comes as North Korean leader Kim Jong Un has insisted that Washington must come around to the notion of undoing some of the sanctions, or else the North will might resume its ICBM tests.

This is the first time the US has seized a North Korean cargo ship.

Media reports from last year indicate Indonesian authorities first stopped the vessel on suspicion of violating sanctions. US officials would not say Thursday if Indonesia turned over the vessel.

The UN Security Council passed a resolution in 2017 banning North Korea from exporting coal.

The move marks an escalation of US government pressure on Pyongyang, even as President Trump has spoken glowingly about North Korean leader Kim Jong Un.

Last month, a federal judge in the District of Columbia approved a set of subpoenas targeting the financial records of Chinese banks which might show how the North Korean regime has sought to evade sanctions over its nuclear program.

In that case, investigators are probing whether a Hong Kong corporation may have helped North Korea evade sanctions.

Kim has reportedly been angered by the lack of progress in talks with Washington, and also by America's decision to re-start military exercises with South Korea.

We imagine the seizure of one of his government's most valuable assets probably won't help calm his nerves.