Mystery has deepened today over the sudden death of two apparently healthy North Korean workers in Moscow as it emerged both had been injected with an unknown 'drug' shortly before they died.

The fatalities in separate rooms of a dormitory-style hotel for guest workers, in the west of the capital, are being investigated by the Russian equivalent to the FBI.

Russia's health watchdog Rospotrebnadzor is also probing the deaths - but so far there have been no official findings.

But Russian broadcaster Ren TV said both North Koreans, believed to be migrant workers, had received injections shortly before they died.

Two North Koreans were found dead in separate rooms in the same Moscow hotel amid reports they both suffered 'acute heart failure'. An ambulance is seen driving near the scene

The men had complained of severe chest pains and shortness of breath before they died on Saturday at a hotel (pictured) in the west of the Russian capital

They appeared to have injected themselves, said the channel.

The drug was evidently 'to cure high body temperature and headache'. But who provided them with the medicine is not yet known.

Foes of North Korean autocrat Kim Jong-un are known in the past to have been poisoned to death and authorities in Russia - which hosts thousands of workers from the repressive state - are investigating the fatalities.

The men died after complaining of severe chest pains and shortness of breath

Two other North Koreans were rushed to an infectious diseases hospital in the capital but no details have been given about their cases. Five other men living at the hotel also complained of illness.

The first man to die was named as Chkhe Men Sen, 37. When paramedics arrived at Gorodskoy Hotel in Yermakova Roscha Street in central Moscow, he was already dead. The inside of one of the hotel's rooms is pictured

The first man to die was named as Chkhe Men Sen, 37.

When paramedics arrived at Gorodskoy Hotel in Yermakova Roscha Street in central Moscow, he was already dead.

Soon afterwards 22-year-old Khon Gim Chkol died as ambulance doctors sought to reanimate him.

The men were in separate rooms.

Two other North Koreans aged 34 and 41 had a 'fever of unknown origin' - and were taken to a Moscow infectious disease hospital.

Initial reports in Russia mentioned a viral infection.

Two other North Koreans aged 34 and 41 had a 'fever of unknown origin' - and were taken from the hotel (pictured) to a Moscow infectious disease hospital. Their condition is not known

Russian health watchdog Rospotrebnadzor as well as police and the Investigative Committee - seen as an equivalent of the FBI - are investigating the cause of the sudden illness

Later reports said the two North Koreans had died from 'acute heart failure'.

North Korean officials have since 'removed' an unknown number of their fellow citizens from the hotel.

Russian health watchdog Rospotrebnadzor as well as police and the Investigative Committee - seen as an equivalent of the FBI - are investigating the cause of the sudden illness.

Pictures posted by immigrant workers from Tajikistan show conditions at the hotel, which is located close to the downtown skyscraper Moscow City district, where there are many building sites.

An employee at the hotel told MailOnline today: 'All questions should be addressed to the lawyers. We don't know anything.'

He refused to give contacts for the relevant lawyers. North Korea has not commented on the deaths.

Two North Koreans have been found dead in separate rooms in the same Moscow hotel amid reports they both suffered 'acute heart failure' (file picture)

It comes just months after the half-brother of North Korean dictator Kim Jong-un was murdered at Kuala Lumpur airport in Malaysia.

Assassins targeted Kim Jong-nam using a deadly nerve agent.

While Malaysia never directly accused North Korea of carrying out the attack, speculation is rampant that Pyongyang orchestrated a hit on a long-exiled member of its ruling elite.

Although Kim, who was estranged from his family, was not an obvious political threat, he may have been seen as a potential rival in the country's dynastic dictatorship.

North Korea has denounced such speculation and has never acknowledged the victim was a member of its ruling family.