The head of Russia’s state-run nuclear corporation has expressed concern about the spread of the new coronavirus to three “nuclear cities”, including one that houses a top-secret research institute that helped develop the Soviet atomic bomb.

The cities are closely linked to Russia’s nuclear industry, which is managed by the Rosatom corporation. Several are closed to foreigners and Russians require special clearance to enter them as facilities located there are closely guarded secrets.

Rosatom chief Alexei Likhachev said special deliveries of ventilators and personal protection equipment (PPE) were being sent to the closed town of Sarov, east of Moscow, and other towns where dozens of cases of the virus have been registered.

“This [pandemic] creates a direct threat to our nuclear towns. The situation in Sarov, Elektrostal [and] Desnogorsk is today particularly alarming,” he said in an online speech to Russia’s nuclear industry workers.

“The situation in Sarov is exacerbated by an outbreak of the illness in the nearby Diveyevo monastery,” he said, without elaborating further.

Likhachev made his remarks on a day when Russia reported its biggest daily rise in new coronavirus cases. Russia now ranks eighth worldwide with 93,558 confirmed cases, though its death toll of 867 is still far below that of many other countries.

Moscow, which accounts for more than half of Russia’s cases, and many other regions have imposed stay-at-home orders to curb the spread of the virus.

Sarov, which was so secret that it did not appear on maps until the breakup of the Soviet Union in 1991, remains an important part of Russia’s nuclear military complex, defence experts say.

It is home to a research institute that gained prominence last year when five of its scientists died in a mysterious accident at a military testing site in the far north.

Rosatom said the incident had occurred during a rocket engine test on a sea platform. Some US experts said they suspected it had been a botched test of a new missile vaunted by the president, Vladimir Putin.

Last week, Rosatom said seven people at Sarov’s All-Russian Scientific Research Institute of Experimental Physics had been diagnosed with coronavirus, bringing the total number of cases in the city – which has a population of about 95,000 – to 23.

It said the outbreak in Sarov had begun when a retired couple returned to the city from a Russian holiday resort and that more than 100 people had since been isolated to stop it spreading further.