Ukraine’s nuclear energy agency has put the last operating reactor at Europe’s largest nuclear power plant into “cold shutdown”.

The move is a safety precaution amid catastrophic flooding from the collapse of a nearby dam, as Russia’s war on Ukraine drags on through its 16th month.

Russian forces have continued pummelling the country with missiles and drones overnight, with Ukrainian officials reporting at least four deaths and damage to a military airfield.

Five out of six reactors at the Zaporizhzhia Nuclear Power Plant, which is occupied by Russian forces, are already in a state of cold shutdown, in which all control rods are inserted into the reactor core to stop the nuclear fission reaction and generation of heat and pressure.

Evacuation in Ukraine
Volunteers haul a woman on a stretcher as she is evacuated from the left bank Dnipro river, in Kherson (AP)

Energoatom, the Ukrainian nuclear agency, said there was “no direct threat” to the Zaporizhzhia plant due to the breach of the Kakhovka dam further down the Dnieper River, which has forced thousands of people to flee flooding and also sharply reduced water levels in a reservoir used to help cool the facility.

The last reactor was put into cold shutdown on Thursday, Energoatom said, adding that other factors in the decision included shelling near the site which has damaged overhead lines connecting the plant to Ukraine’s energy system.

With all nuclear reactions stopped, temperatures and pressure inside reactors gradually decline, reducing the required intensity of water cooling of the radioactive fuel.

This is a nuclear power plant’s safest operating mode. Energoatom employees are still working at the power plant, although it remains controlled by the Russians.

Burning armoured vehicle
A Ukrainian military vehicle being hit during a combat in Ukraine. (Russian defence ministry Press Service via AP)

The site’s power units have not been operating since September last year. The head of the International Atomic Energy Agency (IAEA) is due to visit Ukraine in the coming days.

Analysts and Russia say that Ukraine has launched a counter-offensive in southern Ukraine, potentially aiming to retake territory near the plant.

Also on Saturday morning, Ukrainian authorities reported that at least four civilians have died across the country as Russian forces launched Iranian-made Shahed drones, missiles, and artillery and mortar strikes.

Ukraine’s state emergency service reported that three people were killed and over two dozen wounded overnight in an attack targeting the Black Sea port of Odesa.

Collapsed dam
Water flows over the collapsed Kakhovka dam (AP)

A spokesperson for Ukraine’s southern operational command, Natalia Humeniuk, said two children and a pregnant woman were among those wounded.

In Ukraine’s north-east, a 29-year-old man was killed as more than 10 drones targeted the Kharkiv region, its governor, Oleh Syniehubov, reported.

He added that at least three other civilians were wounded.

In the Poltava region further west, there was damage to a military airfield struck overnight during a Russian drone and missile attack, local governor Dmytro Lunin reported. No-one was hurt.

As of Saturday morning, there was no additional comment from the Ukrainian army or officials on the extent of the damage.

The Ukrainian air force said that during the night, it had shot down 20 out of 35 Shahed drones and two out of eight missiles “of various types” launched by Russian forces.