The BBC quoted an oil industry consultant as saying a drilling ship on contract to international firm Talisman-Vietnam began working in an area of sea about 400 kilometers off the Vietnamese southeast coast on June 21.

The consultant, Ian Cross, of Singapore-based Moyes & Co, told the BBC that drillship Deepsea Metro I is drilling in a piece of seabed known as Block 136-03 by Vietnam and Wan-an Bei 21 by China, which has leased out the same area to a different company.

The BBC said other oil industry sources have said Talisman-Vietnamm, which was formerly owned by the Canadian company Talisman but since 2015 has been part of the Spanish-owned Repsol group, had earlier been denied permission to drill to avoid upsetting China.