KIEV (Reuters) - A helicopter belonging to Ukraine's border guards crashed off an island in the Black Sea on Thursday, killing 12 of 13 people on board, officials said.

Ihor Krol, a spokesman for the Emergencies Ministry, said one passenger aboard the Mi-8 helicopter, a military workhorse in post-Soviet states, had been rescued off Poludenniy island. The crash occurred in marshland in the delta of the river Danube, southwest of the port of Odessa.

"The helicopter was flying from Odessa to Serpent Island," Krol said by telephone. "It (Frankfurt: A0MLX5 - news) was spotted overturned."

A border guards spokesman later said 12 bodies had been found. All but one of those on board were military officers.

The ministry gave no reason for the crash, but a storm warning was in effect for the region.

Serpent Island, a windswept outcrop in the Black Sea, has long been in dispute between post-Soviet Ukraine and Romania. The row has been referred to international courts.

A border guard contingent on the island regularly receives its supplies by air.

(Reporting by Pavel Polityuk; Writing by Ron Popeski)