A forest fire that has raged in Ukraine for more than a week has spread to within a kilometre of the Chernobyl power plant, environmental campaigners have warned.

Footage of the region has shown fires raging through the 30km exclusion zone set up around the site of the worst nuclear disaster in history, with black smoke billowing into the sky as firefighters attempting to beat back the blaze from helicopters.

However, despite state officials assuring the situation is under control, NGO Greenpeace Russia has warned the situation may be much worse than first stated by the authorities.

On 4 April, Ukrainian authorities said the blaze covered an area of 20 hectares, but Greenpeace cited satellite images showing it was around 12,000 hectares in size at that time.

"According to satellite images taken on Monday, the area of the largest fire has reached 34,400 hectares," it said, adding that a second fire, stretching across 12,600 hectares, was just one kilometre away from the defunct plant.

Ukrainian officials did not immediately respond to a request for comment on those claims. The Emergency Situations Service meanwhile said radiation levels in the exclusion zone had not changed and those in nearby Kiev, the Ukrainian capital, "did not exceed natural background levels".

But Rashid Alimov, head of energy projects at Greenpeace Russia, said the fires, fanned by the wind, could disperse radionuclides, atoms that emit radiation.

"A fire approaching a nuclear or hazardous radiation facility is always a risk," Mr Alimov said. "In this case we're hoping for rain tomorrow."

Chernobyl tour operator Yaroslav Yemelianenko, writing on Facebook, described the situation as critical.

He said the fire was rapidly expanding and had reached the abandoned city of Pripyat, two kilometers from where "the most highly active radiation waste of the whole Chernobyl zone is located". He called on officials to warn people of the danger.

The fires, which follow unusually dry weather, began on 3 April in the western part of the exclusion zone and spread to nearby forests.

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