A South Korean vessel carrying sand collided with a North Korean fishing boat in the East Sea Tuesday morning.

A South Korean government official said the 658-ton Dongi-1 collided with a small North Korean fishing boat at 2:35 a.m. some eight kilometers off the coast of the North’s Jangjeon port.

In the accident, two North Korean fishermen went missing and two others were rescued by the South Korean ship.

Owned by a company in Busan, the South Korean vessel delivers sand obtained in the North to the South.

Sand accounts for the largest portion of inter-Korean trade.