Ukraine's biggest steelmaker Metinvest vowed on Friday never to operate under Russian occupation and said that more than a third of the nation's metallurgy production capacity was now out of action due to the siege of the city of Mariupol.

The company, controlled by Ukraine's richest man Rinat Akhmetov, also said that Ukraine had more than halved iron ore output following Russia's Feb. 24 invasion.

Ukraine is one of Europe's biggest suppliers of iron ore.

The figures, which came in a statement to Reuters, laid bare the industrial impact of the war as Russia gears up for a new offensive in the east where Ukraine's steel and coal assets are concentrated.

Metinvest has two vast steel works in Mariupol - Illich and Azovstal - which it put into a special "hot conservation" regime to protect equipment and prevent accidents when the war began.

The city on the Sea of Azov has since been devastated by weeks of shelling and siege.

A dwindling Ukrainian force is holed up there outnumbered and surrounded by a Russian assault. Ukraine's deputy defence minister said fighting was raging around Illich and the port on Friday.

INDUSTRIAL TOLL

Metinvest said in the statement that the sites had been damaged, but that it was impossible to take stock and assess the scale due to the fighting.

It said: "We believe in the victory of Ukraine and plan to resume production after the end of hostilities. Metinvest's metallurgical enterprises will never operate under Russian occupation."

Metinvest's plants in Mariupol accounted for more than a third of Ukraine's entire metallurgical production, it said.

"The country has therefore lost 30-40% of its metallurgical production capacity, since the plants are not working. We have no doubt that their work will be resumed, but for this Mariupol must remain Ukrainian," it said.

Just a week before the war broke out, business magnate Akhmetov had said that Metinvest planned to invest $1 billion this year in modernisation and new production facilities.

On March 19, the company said that two shells had fallen on Azovstal, the steel plant where Ukrainian forces are now holed up in the east of the city.

Just several days before that, it said that shelling had hit the territory of Metinvest's Avdiivka coke plant, damaging some of its facilities.

(Reporting by Natalia Zinets; editing by Tom Balmforth, Guy Faulconbridge)