The Zaporizhzhia plant -- Europe's largest nuclear facility -- has been occupied by Russian troops in southern Ukraine since the opening weeks of the war, and remained on the frontlines ever since.

Recently Moscow and Kyiv have traded blame for shelling around the complex, a "highly volatile" development the International Atomic Energy Agency (IAEA) says "underlines the very real risk of a nuclear disaster".

The development came on a day when Russian President Vladimir Putin signed a decree to increase the headcount of his country's army to more than two million, including 1.15 million servicemen, from next January, according to the document published on a government portal.

Putin last set the army headcount in 2017 at around 1.9 million people with 1.01 million soldiers.

Ukraine state operator Energoatom said the Zaporizhzhia plant was severed from the national network after a power line was twice disconnected by ash pit fires in an adjacent thermal power plant.

The three other power lines "were earlier damaged during terrorist attacks" by Russian forces, the operator said.

"The actions of the invaders caused a complete disconnection of the (Zaporizhzhia Nuclear Power Plant) from the power grid -- the first in the history of the plant," Energoatom added on Telegram.

It added that "start-up operations are under way to connect one of the reactors to the network".

Kyiv officials have said they believe Moscow has seized the station in order to divert power to the Crimean peninsula annexed by Russia in 2014.

Energoatom could not be immediately reached for comment on whether the supply had been diverted, the cause of the ash pit fires, or the number of those without electricity.

However, the mayor of the city of Melitopol Ivan Fedorov said "Russian occupiers cut off the electricity in almost all occupied settlements of Zaporizhzhia".

Earlier on Thursday Britain's defence ministry said satellite imagery from the weekend "indicated that Russia maintained an enhanced military presence at the site".

"The principal risks to reactor operations are likely to remain disruption to the reactors' cooling systems, damage to its back-up power supply, or errors by workers operating under pressure," the ministry added in a statement.

- Independence day deaths -

Meanwhile on Thursday the death toll from an air strike on a train station in central Ukraine rose to 25, as the EU warned those "responsible for Russian rocket terror will be held accountable".

Russia issued a counter-claim saying it targeted soldiers and killed 200 Ukrainian servicemen in the attack Wednesday on a rail hub in Chaplyne city of the Dnipropetrovsk region.

The attack struck six months to the day since Russia began its invasion of Ukraine and also on the day Ukraine celebrated its 1991 independence from the Soviet Union.

On Thursday, state rail operator Ukrainian Railways said the toll had risen overnight from 22 to 25, and included two children with a further 31 people injured.

Ukrainian President Volodymyr Zelensky had warned at the weekend Russia might do something "particularly cruel" to mark Ukraine's independence celebrations.

In a daily press briefing, the defence ministry said the train was "en route to combat zones" in the eastern Donbas region of Ukraine, which Russia seeks to fully control.

But EU foreign policy chief Josep Borrell "strongly" condemned "another heinous attack by Russia on civilians".

"Those responsible for Russian rocket terror will be held accountable," he said on Twitter.

The UN humanitarian coordinator for Ukraine Denise Brown said the strike "is just one more example of the level of suffering that this war is causing the people of Ukraine".

"In the six months since Russia's invasion, Ukrainians have continued to see their loved ones killed, injured and traumatized, their families separated, and their homes, schools and hospitals attacked," she said in a statement.

Meanwhile in Kyiv the city council renamed 102 streets in part of an ongoing effort to "de-Russify" public spaces.

Historic soviet and Russian road names have been replaced with titles lauding Ukrainian culture and commemorating patriots who have fought for the nation over the past six months.