A senior Ukrainian official accused Russia on Sunday of "irresponsible" acts around the occupied Chernobyl power station that pose a radiation threat across much of Europe.
Deputy Prime Minister Iryna Vereshchuk has said Russian forces were "militarising" the exclusion zone around the station, site of the world's worst civil nuclear accident in 1986.
Russia has held the site since the first days of the invasion.
Fears have been growing of a nuclear disaster after Russian troops began shelling the Ukrainian town where staff working at the Chernobyl plant live.
The mayor of Slavutych, the town created and built in the aftermath of the 1986 accident, said on Saturday that Russian forces had taken over the town. Three people were killed in clashes.
There have been concerns that the shelling of the town has been preventing workers coming in and out of the plant.
But the mayor of Slavutych said early on Monday that Russian forces have left after completing their task of surveying it.
"They completed the work they had set out to do," Yuri Fomichev, the mayor of the northern town, said in an online video post.
"They surveyed the town, today they finished doing it and left the town. There aren't any in the town right now."
Ms Vereshchuk is now urging the United Nations to dispatch a mission, in a bid to assess the risks.
Russian forces, she said, were transporting large amounts of old and badly maintained weapons, creating a risk of damaging the containment vessel constructed around the station's wrecked fourth reactor.
And Russian forces were preventing firefighters from bringing under control large numbers of fires in the zone.
"In the context of nuclear safety, the irresponsible and unprofessional actions of Russian servicemen present a very serious threat not only to Ukraine but to hundreds of millions of Europeans," Vereshchuk said on her Telegram account.
"We therefore demand that the U.N. Security Council adopt immediate measures to demilitarise the exclusion zone around the Chernobyl station as well as dispatching a special mission to eliminate the risks of any repeat of the Chernobyl accident resulting from the actions of Russian occupying forces," she said.
Vereshchuk said damage to the containment vessel, built with European financing, would "inevitably lead to the release in the atmosphere of a considerable amount of radioactive dust and contamination not only in Ukraine but also in other European countries".
Russia, she said, was "ignoring these risks" by continuing to transport weapons in areas near the station.
Vereshchuk's claims could not immediately be verified on the ground. Russia has previously denied that its forces have put nuclear facilities inside Ukraine at risk.
The fire and explosion in 1986 in Chernobyl's fourth reactor sent radiation wafting as far away as Britain and Spain.
Thousands of deaths have been linked to the aftermath of the accident and the radiation it released.
All its reactors have now been taken out of service.
Russian forces occupied the Chernobyl station in the first days of the invasion last month and for a time prevented staff maintaining facilities there from leaving or being spelled off by other workers.
The International Atomic Energy Agency said in a statement that it was closely monitoring the situation and expressed concern about the ability of staff to rotate in and out of the station.
Russia's invasion of Ukraine, which it calls a "special military operation", has devastated several Ukrainian cities, caused a major humanitarian crisis and displaced an estimated 10
million people, nearly a quarter of the population.
