Russian soldiers are stationed just meters away from Europe's largest nuclear power plant, satellite images have shown.
The Zaporizhzhia nuclear power plant in southern Ukraine has been held under Russia's control since early March, but recent imagery has shown they have an enhanced presence around the site, western experts have said.
It has been shelled in recent days, raising fears of an accident just 500km from the site of the world's worst nuclear accident, the 1986 Chernobyl disaster.
Both the Ukrainians and the Russians have blamed each other for the shelling.
The last two working reactors at the Russian-held Zaporizhzhia plant were disconnected from the Ukrainian grid on Thursday after nearby fires damaged overhead power lines, Ukraine's state nuclear company Energoatom said.
In a statement on social media the company said: "The actions of the invaders caused a complete disconnection of the [plant] from the power grid - the first in the history of the plant."
UN secretary general Antonio Guterres has called for the area surrounding the reactor complex to be demilitarised.
A statement from the MoD said: "In early March, Russian ground forces assaulted and seized Zaporizhzhia nuclear power plant (ZNPP).
"On 21 August 2022, imagery indicated that Russia maintained an enhanced military presence at the site, with armoured personnel carriers deployed within 60 metres of reactor number five.
"Russian troops were probably attempting to conceal the vehicles by parking them under overhead pipes and gantries. Russia is probably prepared to exploit any Ukrainian military activity near ZNPP for propaganda purposes.
"While Russia maintains the military occupation of ZNPP, 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."
It was claimed earlier this week that staff at the Zaporizhzhia plant have been tortured by occupying Russians to force them to operate the plant since it was captured.
The head of Ukraine's nuclear power industry, Petro Kotin, told Sky News he is hopeful independent international inspectors will be able to visit the site to assess it in the coming days.
He added: "The situation (at the plant) is very bad now and it is worsening all the time. Over the last three weeks, there's been an increase of shelling at the site.
"The actions from Russia increase the danger to nuclear and radiation safety onsite. Also, the conditions of our staff there are really decreasing."
Of the treatment of staff he said: "They captured about 1,100 personnel from the site, and they kept them in their facilities, the captured facilities and police facilities in the (nearby) town of Enerhodar.
"One person was killed, another person was heavily wounded. They're trying to push on them to accept the Russian world. All kinds of psychological pushes on them."
Amid concerns that shelling of the plant could lead to nuclear catastrophe, Western officials have said it was built to withstand most direct military fire.
They said the main concerns are around the nuclear reactors losing water cooling due to a loss of electricity supply.
Having returned from his second holiday this summer earlier this week, Boris Johnson held a call with US president Joe Biden, France’s Emmanuel Macron and German chancellor Olaf Scholz on Sunday to discuss the plant.
The leaders discussed co-operation on international security, including the risk of the fighting around Zaporizhzhia.
They welcomed the Putin’s agreement to allow the UN’s International Atomic Energy Agency (IAEA) to visit the site amid concerns over its safety.
“They stressed the importance of ensuring the safety and security of nuclear installations and welcomed recent discussions on enabling an IAEA mission to the Zaporizhzhia facility,” a No 10 statement said.