Deadly radiation from the Chernobyl nuclear disaster site could be spread through the air by wildfires nearing the contaminated zone in Ukraine.

Emergency crews were battling to halt the progress of blazes which have reached 12 miles from the ruined power plant, inside an exclusion zone designed to protect human health.

A government spokesman in Ukraine called the fires the worst in more than two decades. High winds were blowing the blazes closer to the location of the world's worst nuclear accident, which happened in 1986.

The situation was "under control", according to Prime Minister Arseny Yatseniuk.

Risk is posed by the potential distribution of invisible radioactive isotopes clinging to vegetation such as trees inside the contamination zone. They pose a health hazard because radiation damages human DNA which can cause cancer.

"The situation is being controlled, but this is the biggest fire since 1992. We've not had this scale of fire," a government spokesman said.

"It is around 20km to the plant. Our emergency services are actively working there to prevent the fire spreading further."