* Japan's Mitsubishi Materials on Thursday said that its Indonesia copper smelting unit, PT Smelting, has declared force majeure on deliveries of products to some customers as maintenance at its plant in the country had been extended.
* The smelter on Oct. 31 went into maintenance that was scheduled for about 30 days, but that has been extended due to a lack of oxygen supply from a neighboring oxygen plant, said a Mitsubishi Materials spokesman.
* The oxygen plant was also conducting maintenance that had to be extended, the spokesman said.
* The extended maintenance at the smelter is expected to cut annual production of copper cathode at the unit by about 20,000 tonnes to about 240,000-250,000 tonnes, the spokesman said.
* He did not say how much copper supply would be affected by force majeure or which customers would be hit.
* He said the smelter was expected to resume partial operations in mid-December, returning to full operation in mid-January.
(Reporting by Yuka Obayashi; Editing by Joseph Radford)