UN nuclear chief Rafael Grossi said he discussed the safety of Russia's Kursk nuclear plant with Ukrainian President Volodymyr Zelensky on Tuesday, four weeks after Ukraine launched its unprecedented cross-border incursion.

Ukraine sent thousands of troops into the Kursk region on 6 August, triggering fighting less than 50 kilometres (30 miles) away from the nuclear plant.

Grossi, the head of the International Atomic Energy Agency (IAEA), visited the plant last week, warning that its proximity to ongoing fighting in the Kursk region was "extremely serious".

Travelling to Ukraine this week, Grossi met Zelensky on Tuesday before he was due on Wednesday to again visit the Zaporizhzhia nuclear plant in southern Ukraine, which Russia captured shortly after invading its neighbour in February 2022.

"We had a general evaluation of what I saw (at the Kursk nuclear plant)... For me, it was important to transmit the impressions I got and, of course, to reiterate the importance of avoiding a situation there that would lead to a radiological emergency," Grossi told reporters after meeting Zelensky.

"He was interested in my news... I think it's very clear for him. He understands very well and I don't think he disagrees at all on the fact that nuclear power plants should never be attacked," he added.

Russia has repeatedly sounded the alarm over a possible hit on the plant since Ukrainian troops and tanks stormed into Kursk.

Since Russia invaded Ukraine in 2022, the IAEA has frequently warned of the dangers of fighting around nuclear plants, including of any interruption of power supply due to strikes.

The IAEA on Tuesday said it would send an expert team "soon" to some damaged Ukrainian sub-stations –- "electrical switchyards forming the backbone of the grid -– that have been identified as essential for nuclear safety" to assess the situation.

"The safety of operating nuclear power plants is dependent on a stable and reliable connection to the electricity grid. As a result of the war, the situation is becoming increasingly vulnerable and potentially even dangerous in this regard," Grossi said in a statement.

Last month, the IAEA warned that the safety situation at the Zaporizhzhia nuclear power plant was "deteriorating" following a nearby drone strike.

The plant has come under repeated attacks that both sides have accused each other of carrying out.