Kiev (AFP) - Ukraine said Tuesday only small fires remained in the Chernobyl exclusion zone, with rain coming to the rescue in the battle against a huge blaze.

The fire broke out 10 days ago at the scene of the world's worst nuclear accident in 1986 and on Monday had reached just 1.5 kilometers from the protective dome over the ruined reactor, Greenpeace Russia said.

While forest fires are common in the exclusion zone, the environmental group says that this has been the worst since the 1986 nuclear explosion.

Ukraine's state emergency service said on Tuesday however that the fire was no longer visibly burning and it needed several more days to extinguish the smouldering remains of the blaze.

"Background radiation in the exclusion zone remains within normal limits and is not increasing," the head of the emergencies service, Mykola Chechotkin, told President Volodymyr Zelensky during a conference call.

On Monday the situation had worsened due to stormy wind, but the rain helped firefighters contain the blazes, Chechotkin said.

Chernobyl polluted a large swathe of Europe when its fourth reactor exploded in April 1986. People are not allowed to live within 30 kilometres (18 miles) of the power station.

Police have said the fire was sparked on April 4 by a man burning dry grass near the exclusion zone around the ruined reactor. The flames spread quickly, fanned by strong winds.

Kiev has mobilised helicopters and more than 400 firefighters, with planes dropping tonnes of water on the fire.

Zelensky said late on Monday he was closely following the situation and was "grateful for the courage" of the firefighters.

Oleksandr Syrota, head of the Chernobyl information centre, welcomed reports of rain in the exclusion zone as "good news".

Interior ministry spokesman Artem Shevchenko said rain "has greatly helped rescuers who have been fighting the fire in the area for over a week".

- 'Unpredictable' -

On Monday, Sergiy Zibtsev, head of the Regional Eastern European Fire Monitoring Center, told AFP that the fire was "super-huge" and "unpredictable".

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"In the west of the exclusion zone it has already covered 20,000 hectares (50,000 acres) by our calculations."

The Ukrainian emergencies service has not provided recent figures on the size of the fire, but said that "there is no threat to the Chernobyl nuclear power plant and the storage facilities".

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

After the 1986 explosion, 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.