Firefighters in the Chernobyl exclusion zone of northern Ukraine on Monday entered their third day of battling a forest fire in the area, still contaminated with radiation from the nuclear power plant disaster more than three decades ago, dpa reported. Radiation in the area was 16 times higher than normal background levels, a senior environmental official, Egor Firsov, said in a statement. About 25 hectares of forestland within the predominantly uninhabited area were ablaze on Monday morning, the State Emergency Service said in a statement. No victims have been reported. About a fourth of the blaze was in the Chernobyl zone. The blaze was reported to have spread to an area of more than 100 hectares over the weekend. About 140 firefighters have been working to extinguish the blaze. Firsov, who heads a state environmental watchdog, said the blaze is believed to have developed from a grass fire. Grass fires are common in the early spring as farmers conduct controlled burnings to remove brush. An investigation is under way. The 1986 reactor meltdown and explosion at the Chernobyl nuclear power plant, about 100 kilometres north of Kiev, is considered the worst nuclear disaster in history.