Four people have been arrested in connection with a Ukrainian nursing home fire that has killed at least 15 people.

Thursday's blaze tore through the second floor of an illegal residential home for the elderly in the eastern Ukrainian city of Kharkiv.

Ukraine's Prosecutor General Iryna Venediktova said the four people detained included the building's owner, the tenant and his wife - the care home's director. The home's administrator was also arrested.

Investigators are looking into three potential causes of the fire: arson, misuse of electrical devices and a short circuit.

President Volodymyr Zelensky visited the scene of the tragedy on Friday morning, announcing a national day of mourning on Saturday for the victims.

Nine people were rescued from the flames, five of whom remain in hospital.

"The fire spread very quickly and all the people on the second floor were less mobile or had illnesses related to mental disorders," First Deputy Minister of Internal Affairs Sergiy Yarovyi told Zelensky at the scene.

Zelensky said it was vital for authorities to inspect similar facilities across Ukraine and said police have found six such places in Kharkiv, the country's second city. He called it a "very serious challenge" for the state.

Authorities confirmed the care home was not officially registered and was operating illegally. It charged residents more than $480 per month, a large sum for Ukraine, where the average monthly salary is $428.

Deadly fires are not uncommon in Ukraine, where compliance with safety regulations is poorly enforced and ageing infrastructure is badly maintained.

In December 2019, a fire killed 16 people and injured 30 others at a higher education institution in the Black Sea city of Odessa. The director was later arrested on charges of negligence.

Four months earlier, a fire in a hotel in Odessa killed nine people.