"There is bad news -- radiation is above normal in the fire's centre," Yegor Firsov, head of Ukraine's state ecological inspection service, said on Facebook.

The post included a video with a Geiger counter showing radiation at 16 times above normal.

The fire has spread to about 100 hectares (250 acres) of forest, Firsov wrote.

Kiev has mobilised two planes, a helicopter and around 100 firefighters to fight the blaze, which broke out Saturday and spread over 20 hectares in a forested area near the Chernobyl power plant.

On Sunday morning, the fire was not visibly burning and no increase in radiation in the air had been detected, the emergencies service said in a statement.

However, the service said Saturday that increased radiation in some areas had led to "difficulties" in fighting the fire, while stressing that people living nearby were not in danger.

Chernobyl polluted a large swathe of Europe when its fourth reactor exploded in April 1986, with the area immediately around the power plant the worst affected.

People are not allowed to live within 30 kilometres (18 miles) of the power station.

The three other reactors at Chernobyl continued to generate electricity until the power station finally closed in 2000. A giant protective dome was put in place over the fourth reactor in 2016.

Fires are common in the forests near the disused power plant.