A hundred technicians are working under armed guard to maintain the defunct Chernobyl nuclear plant in northern Ukraine, held hostage for the last three weeks by Russian forces who seized the compound within the first hours of the invasion.

Tired and poorly fed, they were working the night shift when Russia captured the site of the 1986 core meltdown that sparked the worst nuclear reactor catastrophe in history.

Relatives and colleagues contacted by Agence France-Presse (AFP) claim the crew members have been unable to return to their homes in nearby Slavutych, the city built to house Chernobyl workers after the disaster.

"Physically and morally, they are exhausted," said the wife of one technician, who like others at the site can communicate with the outside world only via telephone.

"They think that no one cares about them, neither the Russian government nor the Ukrainian government," she said, adding that they are getting only two small meals a day.

"They can take a shower, but with no soap, no shampoo, they can't brush their teeth. They can't change their clothes or wash them. There is no supply of medicines. They are sleeping on the floor, on some desks or on chairs."

Around 100 other people, including security personnel, are also being detained at the site.

It is unclear why Russian soldiers seized Chernobyl, where the destroyed reactor is kept under close supervision within a concrete and lead sarcophagus, and the three other reactors are being decommissioned.

In 2017, the site was one of several Ukrainian targets hit by a massive cyberattack thought to have originated in Russia, which briefly took its radiation monitoring system off-line.

On Sunday, several dozens of people, including women and children, held a protest in Slavutych over the treatment of personnel at the plant and the potential safety risks.

The electricity supply to Chernobyl was halted on several occasions since the Russian takeover.

"Our boys are not just hostages but prisoners in a Russian concentration camp," one woman at the protest told local television.

The head of the International Atomic Energy Agency (IAEA), Rafael Grossi, said Tuesday that the Chernobyl technicians and guards were being forced to work "under enormous stress without the necessary rest."

To ensure against radioactive risks, "operating staff must be able to fulfill their safety and security duties and have the capacity to make decisions free of undue pressure," he warned.

A Chernobyl engineer told AFP that the employees are "deeply worried that they will be on the front line if an accident happens."

The pool where the spent fuel is stocked is "overpacked by 40%" she added, and "backup pools should be empty but they are also filled with other spent fuel. This situation is against international nuclear safety regulations."

Contacted by AFP, officials at Ukraine's atomic energy agency were unavailable to comment on the claims.

Russian forces also shelled and captured the Zaporizhzhia atomic power plant, Europe's biggest, on March 4, causing a fire that raised alarm in Europe over a possible nuclear catastrophe.

Karine Herviou, deputy director-general France's IRSN nuclear safety watchdog, said, "there is no risk of an explosion at the site."

"Unlike at nuclear plants that are in operation, a sustained loss of electricity supply to the site will not cause an accident," she said.

But the risks of war remain, with the relative of one technician saying that Russia has effectively built "a military base" at Chernobyl complete with missile-launching batteries.

"The strategy is brilliant on the war side, but for humanity, it is absolutely insane – no one will fire a missile on Chernobyl to destroy" Russian forces, said the relative, himself a former employee at the site.

He said the chances of a disaster were high, not least because of alleged safety breaches by Ukrainian authorities – which he claims to have seen first-hand – and because the soldiers guarding the employees "don't know what's going on."

"In nuclear safety, you always try to forecast the worst scenario and try to avoid it. Right now, they are trying to hide it, like the USSR did in 1986," the relative said.