Austrian energy company OMV and U.S. counterpart, Exxon Mobil, said Monday drilling started into a new reserve prospect off the Romanian coast.

Both companies announced the start of operations at a new prospect dubbed Neptun in the Romanian waters of the Black Sea.

Gabriel Selischi, an exploration and production director at OMV, said the Black Sea plays a central role in its offshore ambitions and results so far have been positive.

"However, much of the activity in the Black Sea deepwater area is of a frontier, pioneering nature, involving high investment risks and therefore requiring a stable investment framework," he said in a statement.

Romania has some of the largest proved oil reserves in Europe, though production has been in a steady decline.

The Neptun block lies in deep Romanian waters. OMV and Exxon signed a partnership agreement for exploration and production in the region in 2008.