Authorities said radiation levels in Kyiv and its surroundings were normal.

Ukrainian authorities have sought to calm fears around a forest fire burning in the contaminated zone around the Chernobyl nuclear power station that briefly caused local radiation levels to rise.

Firefighters on Monday said they were still trying to extinguish two fires that had begun on Saturday and which had spread to part of the 30-mile “exclusion zone” around the power station, the scene of the world’s worst nuclear disaster in 1986.

Around 100 firefighters, backed by planes and helicopters, were deployed, a day after they succeeded in putting out part of the fire at another nearby site.

The fires have sparked fears about radiation in Ukraine’s capital, Kyiv, which is about 60 miles south of Chernobyl, but authorities said that testing by experts sent by the government on Monday had found that there had been no rise in radiation levels in Kyiv or its surrounding suburbs.

“You needn’t be afraid to open your windows or to air out your apartment during quarantine,” Yegor Firsov, the head of Ukraine’s state ecological inspection service wrote on his Facebook page on Monday.

Ukraine’s state emergency service said that by Monday evening around 20 hectares (50 acres) were still on fire inside the exclusion zone. Video posted by the service Monday showed a helicopter flying over the fire and firefighters on the ground spraying water. A day earlier, the service said part of the fire had been extinguished near the villages of Volodymyrivka and Zhovtneve, close to the zone’s edge.

Police said they had tracked down a 27 year-old man they suspected had started the fire by igniting long grass in the area. The man had told them that he had set fire to some garbage and grass “for fun” but that. fanned by the wind. the fire had quickly got out of control and he had been unable to extinguish it, police said in a statement.

The fires attracted international attention after Firsov, the head of the ecological inspection service, published a post on Sunday warning that radiation levels at the heart of the fire had risen 16 times above the norm.

“There is bad news—radiation is above norm at the center of the fire,” he wrote on Sunday. He included a video showing a Geiger counter beeping with a reading of 2.3 micro sieverts per hour. He wrote that it was it difficult for firefighters to put out the blaze.

Firsov blamed the fire on what he called the “barbaric” practice of burning grass and urged lawmakers to introduce legislation that would significantly increase fines for causing forest fires.

The state emergency service issued an air pollution warning for Kyiv on Monday, but said it was related to weather conditions and not the fires, stressing that radiation levels were normal. It noted that gamma radiation levels around the fires had not risen.

Forest fires near Chernobyl are common and have occurred for the past three years in a row. The exclusion zone has existed around the power station since April 1986 when its fourth reactor exploded, spreading radioactive pollution across Europe. The station’s other three reactors continued to provide electricity until 2000, when they were shut down.

In 2016, a giant stadium-sized dome was moved over the destroyed fourth reactor to replace a concrete shield known as the “sarcophagus” that had been erected following the accident and that was decaying.

Although some hotspots remain, radiation levels in most of the exclusion zone are not above normal and it has effectively become a nature reserve. Organized tourism at the site has boomed since last year’s HBO mini-series “Chernobyl.”