North Korean ships continue to pass through the South Korean port of Incheon without difficulty, despite recent tension between the two Koreas.

The North recently expelled South Korean officials working at an inter-Korean economic cooperation office at the Gaesong Industrial Complex.

It also test-fired a series of short-range missiles over the West Sea.

The Incheon Regional Maritime Affairs and Port Office said three North Korean ships have docked at the South Korean port five times this month.

North Korean vessels began regular use of Incheon port in May last year, about two years after an inter-Korean maritime agreement went into effect in 2005.

They leave North Korean port cities such as Nampo and Haeju, bringing coal, glass and mushrooms. The ships carry construction materials and aid goods when they return home.