"The fire is at a distance of 15 to 20 kilometres from Chernobyl," Maya Rudenko, a spokeswoman for the plant, told AFP by telephone, adding there was "no problem" there.
"The situation is under control," the head of the emergency services, Zoryan Shkiryak, said on Facebook.
Interior Minister Arsen Avakov warned however that "the high flames and sudden gusts of wind mean there is a serious risk that the fire could spread."
Avakov said the fire had spread over some 400 hectares near the site, which lies around 100 kilometres (62 miles) from the Ukrainian capital Kiev.
More From This Section
Around 200 firefighters with scores of vehicles were mobilised to battle the inferno and aircraft were dumping water on the flames, the state emergency services said.
They said the fire started shortly after noon (around 0915 GMT).
Just two days earlier 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.
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.
They hastily laid over the reactor site a concrete cover dubbed "the sarcophagus", which is now cracking and must be replaced.
The work had been scheduled for completion by the end of this year but officials have said that technical problems will delay it until late 2017.