According to the Member of the State Agency of Ukraine on Exclusion Zone Management Yaroslav Yemelianenko, the bushfires have reached the town of Pripyat, adding that the flames were now two kilometers away from nuclear power station and its destroyed fourth reactor.
According to the Ukrainian State Emergency Situations Service (SES), over 300 firefighters backed by 83 units of equipment, including two planes and three helicopters, are involved in the firefighting operation.
The SES 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 Greenpeace Russia said the situation is much worse than Ukrainian authorities believe, and that the fires cover an area one thousand times bigger than they claim.
On April 4, 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 kilometer away from the defunct plant.
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," 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 April 3 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. It remains unclear if the person, who has reportedly confessed to starting a number of fires "for fun," is partly or fully responsible.