Another four people are being treated for severe burns following the blaze in Kosiv, Western Ukraine, on Tuesday.
According to officials, it began when a candle was lit by a member of staff near five oxygen concentrators – which instantly caught fire.
It is the latest in a number of deadly hospital fires during the pandemic.
Pure oxygen in cylinders which are commonly found in hospitals – and have been heavily relied on to treat Covid-19 patients – is highly flammable.
Local emergency services said that “ignorance of the elementary laws of physics and disregard for safety rules have led to irreparable losses”.
The local prosecutors office is investigating the incident, Interfax news agency reported. According to a statement, the people who died were two women and a man, while three doctors suffered burns.
Oxygen cylinders are believed to have been a factor in a number of hospital fires during the Covid-19 pandemic – both in Ukraine and elsewhere.
In February, there were two deadly hospital fires in Ukraine.
In September, at least 14 people were killed in North Macedonia after fire engulfed a makeshift hospital treating coronavirus patients. At the time, officials said they suspected oxygen cylinders had exploded.
Source: BBC News