"This is a scheduled maintenance, and has nothing to do with the recent shutdown," the source said, referring to a shutdown earlier this month.

The source said, however, that the maintenance plan is tentative and subject to change.

The 200,000-barrel-per-day refinery, located 260 km (160 miles) south of Hanoi, is 35.1% owned by Japan's Idemitsu Kosan Co, 35.1% by Kuwait Petroleum, 25.1% by PetroVietnam and 4.7% by Mitsui Chemicals Inc.

On Tuesday, an executive of Binh Son Refining and Petrochemical said its 130,000-bpd Dung Quat refinery would shut down for 51 days starting from early June next year for a major maintenance.

(Reporting by Khanh Vu and Jessica Jaganathan; Editing by Kim Coghill)