The United States has told any of its citizens currently in North Korea to leave.

The US State Department is to ban US travel to the country from Friday September 1st.

The ban will restrict the use of US passports to travel into, in, or through the country.

The US says any of its citizens currently in North Korea should leave before the travel restriction comes into effect.

A statement says the US Secretary of State Rex Tillerson has authorised the restriction "due to the serious and mounting risk of arrest and long-term detention of US citizens under North Korea’s system of law enforcement".

Anyone who wants to travel to North Korea on a US passport must obtain a special validation, which will be granted "only under very limited circumstances."

It follows the death of US student Otto Warmbier a week after his release from a North Korean prison.

He was sentenced to 15 years in prison with hard labour in 2016, after he admitted trying to steal a propaganda sign from the staff-only area of a hotel he was staying at.

Some tour companies offer trips to the reclusive state.

There is no US embassy or consulate in North Korea. US interests there are represented by the Swedish embassy.

The Irish Department of Foreign Affairs says citizens should avoid non-essential travel to North Korea.