UN inspectors headed into Ukraine’s battlefield on Wednesday to visit the Zaporizhzhia power plant after warnings that fighting near the nuclear facility risked catastrophe.

Russian-installed officials in Enerhodar, the occupied town neighbouring Zaporizhzhia, claimed that Ukrainian forces fired within the plant’s grounds overnight.

The two sides have spent weeks trading accusations of reckless bombing in the vicinity of the plant claimed by Russia in the early days of a six-month war.

For weeks now, Ukraine and Russia have accused each other of endangering the plant’s safety with artillery or drone strikes and risking a Chernobyl-style radiation disaster.

“We are going to a war zone. We are going to occupied territory,” Rafael Grossi, the head of the UN’s International Atomic Energy Agency (IAEA), said as his team headed on the 280-mile journey southeast from Kyiv.

“This is the first time, it’s the first time of anybody’s going to cross the front line,” he said, adding that he had received “explicit guarantees” from Russia that the mission of 14 experts would be able to work in the occupied plant.

Though Russian forces have taken control of the plant, Ukrainian workers keep it running.

Tass, a Russian state news agency, said the IAEA team was expected to arrive on Thursday morning. Russian-installed officials in the area near the power station suggested the visit might last only one day, while IAEA and Ukrainian officials suggested it would last longer.

“The mission will take a few days. If we are able to establish a permanent presence, or a continued presence, then it’s going to be prolonged. But this first segment is going to take a few days,” Grossi told reporters at a hotel in Zaporizhzhia.

Fighting was reported both near the power station and further afield, with Kyiv and Moscow both claiming battlefield successes as Ukraine mounted a counter-offensive to recapture territory in the south.

Zaporizhzhia is a vital source of energy for Ukraine as the most significant of the country’s four nuclear power stations, which collectively provide around half of Ukraine’s electricity.

Shelling near Zaporizhzhia last week resulted in the plant being cut-off from the electrical grid because of fire damage, causing a blackout in the region and heightening fears of a catastrophe in a country haunted by the Chernobyl disaster.

Energoatom, Ukraine’s energy watchdog, warned that Russian attacks had damaged the plant’s infrastructure and risked “hydrogen leakage and sputtering of radioactive substances”.

Urkainian energy minister German Galushchenko said Kyiv was seeking international assistance to try and demilitarise the area.

“We think that the mission should be a very important step to return [the plant] to Ukrainian government control by the end of the year,” Mr Galushchenko told the Associated Press.

“We have information that they are now trying to hide their military presence, so they should check all of this.”

International bodies have expressed concern over the risk from fighting by the nuclear plant.

The United Nations has called for a withdrawal of military equipment and personnel from Zaporizhzhia to ensure it is not a target while a G7 joint statement called for Russia to immediately return the plant to Ukrainian control.

The Kremlin has ruled out a retreat.

News of the inspections came as small amounts of radioactive isotopes have been detected in air samples collected in Kotka in south-east Finland, but there is no risk to humans and similar discoveries are “quite normal”, the country’s nuclear safety watchdog said on Wednesday.

“The observed radioactivity has no impact on the environment or human health, as the concentrations were very low,” the Radiation and Nuclear Safety Authority (STUK) said in a statement.

Finland, Sweden, Russia and the wider region have a number of nuclear power reactors.

“Small amounts of zirconium and niobium can escape into the air, for example, from the fuel of a nuclear power plant during plant maintenance,” STUK said.

During the collection of the sample, air currents were flowing to Kotka from the east and southeast, but the source of the radiation was unknown, it added.