Six people were killed in a fire at a municipal psychiatric clinic in the Ukrainian southern port of Odessa, authorities said on Tuesday.

The fire erupted late Monday evening in the one-story building, Ukraine's Emergency Service said in a statement.

"Six people died and four have been hospitalized," the statement said. The cause of the fire remains unclear.

Squads of firefighters, emergency and medical workers and policemen were seen at the site, extinguishing the blaze and evacuating patients. 

Nine other people escaped unharmed.

Ukrainian President Volodymyr Zelenskiy expressed his condolences. "The reasons (for the fire) will be established. I present my condolences to the families of the victims," he said on Twitter.

A criminal investigation has been launched into the incident.

(With input from Reuters)

(Cover image: The scene after a fire occurred at a municipal psychiatric clinic in the Black Sea port of Odessa, Ukraine, June 11, 2019. /VCG Photo)