Firefighters have stopped a forest fire near Ukraine's Chernobyl plant, the scene of the world's worst civil nuclear disaster.

The inferno started yesterday at midday local time and spread across some 320 hectares, coming within 20 metres of the site.

More than 300 firefighters with scores of vehicles battled the blaze on the ground, while aircraft dumped water onto the flames from above.

"At 6:00am local time, the fire was controlled and stopped," emergency services said in a statement.

"Work is continuing to extinguish the fire."

Officials said several people who had been living as squatters in the area were evacuated for their safety.

There are no reported deaths from the fire.

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The cause of the blaze is not confirmed, but Ukraine's interior minister Arsen Avakov said the investigation had not ruled out arson.

The area around Chernobyl was evacuated after the 1986 blast and the last reactor there was shut down in 2000, but some personnel still operate in the exclusion zone, where work is underway to lay a new seal over the reactor site.

Radiation level has not changed: official

A state nuclear inspection official said "the level of radiation at the Chernobyl plant has not changed".

The fire comes just two days after Ukrainians marked 29 years since the Chernobyl nuclear disaster.

The explosion of reactor number four on April 26, 1986, spewed poisonous radiation over large parts of Europe, particularly Ukraine, Belarus and Russia.

The human toll of the disaster is still disputed.

People lay flowers in front of portraits at the monument to Chernobyl victims, 29 years after the world's worst nuclear accident. ( AFP: Genya Savilov )

United Nations experts officially recognised 31 deaths among plant workers and firefighters directly linked to the blast.

But environmental group Greenpeace has suggested there could be around 100,000 additional deaths from cancer caused by the disaster.

The Soviet authorities of the time dispatched hundreds of thousands of people to put out the fire and clean the site, without proper protection.

They hastily laid over the reactor site a concrete cover dubbed "the sarcophagus", which is now cracking and must be replaced.

Ukraine's president Petro Poroshenko on Sunday inspected ongoing work on a new 20,000-tonne steel cover - a project estimated to cost more than two billion euros ($2.7 billion).

It is financed by international donations managed by the European Bank for Reconstruction and Development (EBRD).

The structure will contain technology that will act beneath the cover to decontaminate the area once the steel layer is in place. Officials say the new cover will last for 100 years.

The project had been scheduled for completion by the end of this year but the EBRD said last year technical problems would delay it until late 2017.

AFP