Local police said the accident happened on Sunday when several people working on a construction site ignored barriers and warning signs on the beach and went swimming in the sea in the Belhorod-Dnistrovskyi district.

"In the water, as a result of an explosion of an unknown object, three men aged 25, 32 and 53 years old ... were killed," the police said in a written statement.

It added that another man and a woman had been wounded.

This summer Ukrainian authorities closed sea beaches because of mines or artillery shells because of the war with Russia and shelling by Russian forces.

Ukraine's military also planted mines along the coast in case of a Russian amphibious assault after Moscow's February 24 invasion, cordoning off beach entrances with red and white tape to ensure civilians don't get hurt.

"We emphasise once again: that swimming in the reservoirs of the Odesa region is now dangerous and prohibited," the regional police said. "The beaches and coasts of the region are protected and equipped with explosive devices."

Australian Associated Press