Five North Koreans working on a building site in the Russian Far East have been found dead, apparently after inhaling fumes from a diesel-powered generator, investigators said on Monday.

The North Koreans were among the several thousand citizens of the secretive Stalinist state believed to be working around the city of Vladivostok in the Russian Far East, which has a small land border with North Korea.

"The preliminary cause of death appears to have been carbon monoxide poisoning from a diesel generator in a neighbouring room," the Russian Investigative Committee said in a statement.

Russian news agencies said that the five were discovered in the morning and were apparently overcome by poisonous fumes while sleeping in a temporary unit on a building site in the city of Vladivostok.

But the ITAR-TASS news agency said it was possible that the door to their room was locked from the outside and they could not escape.

Some 10,000 North Koreans are believed to work in the Russian Far East, mainly in the timber sector, in line with past agreements between Moscow and Pyongyang.

Migrants from the former Soviet Union and its allied Communist states are regularly employed on building sites in Russia in conditions that activists complain violate their human rights. (AP)