The North Korean coast guard has opened fire on a Russian cargo ship in the Sea of Japan without issuing the necessary warning.

None of the people onboard the Russian fishing schooner was injured and the Russian vessel was undamaged, said the press service of the Federal Fisheries Agency in Russia.

"The Russian fishing schooner Altai en route from the port Zarubino with live crab cargo onboard to the South Korean port of Donghae came under fire from automatic firearms and pistol flare lights at 18:30 Moscow time (14:30 GMT) on 20 September from a warship of the Korean People's Democratic Republic (DPRK)," said a statement.

After resorting to firing, armed North Korean personnel dressed in military uniform boarded the Russian ship to inspect the cargo, it has been reported. The men questioned the vessel's captain for several minutes.

The fishing company said: "Altai was searched through and the captain was questioned about the port of destination, cargo and the line-up of crew. Armed people left the Russian vessel at 18:50 Moscow time (14:50 GMT), the ship was permitted to sail to the destination port."

The statement added: "The North Korean warship did not get in radio contact."

The company's spokesperson told the state-run RIA Novosti that the North Korean coast guard's action was a gross violation of international law on maritime traffic.

Moscow had earlier warned North Korea of dire consequences after reports emerged that the defiant nation had restarted a nuclear reactor.