Police said they tracked down a person suspected of starting the blaze - the 27-year-old man said he burned grass "for fun", and then failed to extinguish the fire when the wind caused it to expand quickly.
Kyiv has mobilised two planes, a helicopter and about 100 firefighters to fight the blaze, which broke out on Saturday and spread over 20 hectares (49 acres) in a forested area near the Chernobyl power plant. It said the other fire was about 50 acres.
In April, 1986, the explosion in a nuclear reactor in Chernobyl led to widespread contamination and wreaked havoc.
Rescuers continue the firefighting operations on the area of around 25 hectares in the Kotovske forestry, Chornobyl Exclusion Zone, near the villages of Volodymyrivka, Zhovtneve and Rahivka using aviation, the radiation background in Kyiv and region is within normal range, the State Emergency Situations Service of Ukraine has said.
The blaze caused radiation levels in the exclusion zone to surge with some readings showing radioactivity at nearly five times the permitted levels.
Firsov said ignorant citizens likely started the blazes by setting fire to grass in the area.
Another reading taken near the fire showed 0.33 μSv/h, which is still high but below the maximum safe amount of 0.5 μSv/h under Ukrainian guidelines.
As CNN reports, the blaze began Saturday inside Chernobyl's exclusion zone, which encompasses an area measuring 2,600 square kilometres. The zone is largely unpopulated, although about 200 people have remained there despite orders to leave.
The Chernobyl power plant ceased all operations in 2000, and its ruined reactor has since been encased in a massive protective dome.
The authority also clarified misinformation by some media that radiation levels had changed in the country's capital of Kiev, which is 100 miles from the Chernobyl exclusion zone. Normally, the background radiation in the area is 0.14 μSv/h, he added.