"The occupiers dug in many places. They buried heavy equipment, created dugouts, even underground kitchens, tents, fortifications," said Yevgen Kramarenko, head of the agency for the Chernobyl exclusion zone.

"One such fortification was located near a site for the temporary storage of radioactive waste" in an area known as the "Red Forest", he told a briefing.

"The system for monitoring radiation in the exclusion zone is still not working," Kramarenko also said, adding that "the servers that processed this information have disappeared".

"We cannot currently say if it is completely safe.

"Until electricity is available and workers receive permission from the armed forces to visit radiation monitoring points, we will not understand how much damage has been done," he added.

Kramarenko said he believed Russian soldiers would feel the consequences of exposure from digging and from the dust clouds created by heavy equipment "very soon".

"Some may in a month, some in years," he said.

He said around 1,000 Russian soldiers were based in the area over a period of several weeks and about 50 armoured cars were taken there.

The Chernobyl power station was the site in 1986 of the world's worst nuclear disaster.