Forest fires are raging in a contaminated area near the defunct Chernobyl nuclear plant, but Ukrainian officials insisted there is no radiation threat.

Hundreds of firefighters backed by aircraft have battled several forest fires around Chernobyl since last week. They managed to contain the initial blaze, but new fires flared close to the decommissioned plant on Tuesday.

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Environmental activists warned on Monday the fire, near the site of the world's worst nuclear disaster in 1986, posed a radiation risk.

Volodymyr Demchuk, a senior official from Ukraine's emergency service, said the situation was under control.

"There is no threat to the Chernobyl nuclear power plant, waste fuel storage or other critical facilities," he said in a video statement late on Monday.

The emergency service said radiation levels in the capital, Kyiv, about 100km (60 miles) south of the plant, were within norms.

President Volodymyr Zelenskyy has promised transparency on the issue and will meet the head of the emergency service later on Tuesday. "Society must know the truth and be safe," he said in a statement late on Monday.

The fire, one of several which followed unusually dry weather, began on April 3 in the western part of the exclusion zone and spread to nearby forests.

'Critical situation'

Police say they have identified a 27-year old resident who they accuse of deliberately starting the blaze.

It remains unclear if the person, who has reportedly confessed to starting several fires "for fun", is partly or fully responsible.

The flames spread quickly, fanned by strong winds, and Kyiv mobilised helicopters and more than 400 firefighters, with planes dropping tonnes of water on the fire.

Yaroslav Yemelyanenko, head of the Chernobyl tour guide association, said one fire was raging within 2km (about 1.2 miles) from one of the radioactive waste depots.

He said the fire had reached the ghost town of Pripyat, a city near Chernobyl whose population of about 50,000 was evacuated after the explosion.

“The situation is critical,” Yemelianenko wrote on his Facebook page.

A Geiger counter shows increased radiation levels against the background of the forest fire. [Yaroslav Yemelianenko/AP]

While forest fires are common in the exclusion zone, Greenpeace Russia said on Monday this was the worst since the 1986 nuclear explosion.

The environmental campaign group said that analysis of satellite images showed the fire at its closest point was just 1.5km (less than a mile) from the protective dome over the ruined reactor.

Government agencies insist the fire has not caused a spike in radiation levels.

Chernobyl polluted a large swath of Europe when its fourth reactor exploded in April 1986. 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.