JAKARTA, April 20 (Reuters) -- Indonesia's Pertamina will select a development partner for the $12-billion Tuban refinery project by the end of April, around two months later than initially estimated, a director at the state-owned energy company said late on Tuesday.

Pertamina is currently in talks with Saudi Arabian national oil giant Saudi Aramco, China's Sinopec Corp., Kuwait Petroleum International (KPI), Russia's Rosneft and a consortium involving Thailand's PTT Global Chemical and Thai Oil, said Pertamina refineries director Rachmad Hardadi.

"We expect a firm partner will have been decided by the end of April," Hardadi said, adding that a sixth contender for the partnership, Indian Oil Corp., had been eliminated.

Pertamina had initially expected to finalize a partner by February. Hardadi did not give a reason for the delay.

Indonesia's fuel output has been hit by a lack of investment in its refining sector since the last refinery was completed in 1994. Pertamina currently has 1 MMbpd in refining capacity, which it plans to raise to 2.3 MMpd through upgrades and additional plants.

The new Tuban refinery, which will eventually be integrated with the TPPI petrochemical refinery that Pertamina took over in late 2015, will have a crude processing capacity of 300,000 bpd and is targeted for completion in 2021.