LVIV: Russian forces seized Europe’s biggest nuclear power plant on Friday in what Washington called a reckless assault that risked catastrophe, although a blaze in a training building was extinguished and officials said the facility was now safe.

Combat raged elsewhere in Ukraine as Russian forces surrounded and bombarded several cities in the second week of the assault launched by Russian President Vladimir Putin.

A presidential adviser said an advance had been halted on the southern city of Mykolayiv after local authorities said Russian troops had entered it. If captured, the city of 500,000 people would be the biggest yet to fall.

The capital Kyiv, in the path of a Russian armoured column that has been stalled on a road for days, came under renewed attack, with air raid sirens blaring in the morning and explosions audible from the city centre.

The US Embassy in Ukraine called the Russian assault on the Zaporizhzhia plant a “war crime”. Pentagon spokesperson John Kirby said it showed how reckless the Russian invasion has been.

Video verified by Reuters showed one building aflame and a volley of incoming shells before a large incandescent ball lit up the sky, exploding beside a car park and sending smoke billowing across the compound.

Thousands of people are believed to have been killed or wounded and more than 1 million refugees have fled Ukraine since February 24, when Putin ordered the biggest attack on a European state since World War Two.

Although the nuclear plant was later said to be safe and the fire out, officials worried about the precarious circumstances.

International Atomic Energy Agency chief Raphael Grossi paid homage to the plant’s Ukrainian staff: “to their bravery, to their courage, to their resilience because they are doing this in very difficult circumstances.”

The plant was undamaged from what he believed was a Russian projectile, Grossi said. Only one of its six reactors was working, at around 60% of capacity.

An official at Energoatom, the Ukrainian state nuclear plant operator, told Reuters there was no further fighting and radiation was normal, but his organisation no longer had contact with the plant’s managers or control over its nuclear material.