Ukrainian authorities have reported a spike in radiation levels in the restricted zone around Chernobyl, the scene of the world's worst nuclear accident, caused by a forest fire.

"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 on Sunday.

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

The fire has spread to about 100 hectares (247 acres) of the forest, Firsov wrote.

Kyiv has mobilised two planes, a helicopter and about 100 firefighters to fight the blaze, which broke out on Saturday and spread over 20 hectares (49 acres) in a forested area near the Chernobyl power plant.

A Geiger counter shows increased radiation level against the background of the forest fire burning near the village of Volodymyrivka, in the exclusion zone around the Chernobyl nuclear power plant [Yaroslav Yemelianenko/The Associated Press]

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

However, increased radiation in some areas had led to "difficulties" in fighting the fire, it said, 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 30km (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.