JAKARTA, Nov 17 (Reuters) - Greenpeace vowed on Saturday to keep up a blockade on a key palm oil port in Indonesia's Sumatra as part of protest against forest destruction, as a tanker it had been trying to block for three days managed to leave the port.

The group's Rainbow Warrior ship dropped anchor next to the MT Westama in Dumai port in Sumatra island on Thursday in a bid to prevent the ship from leaving for India.

The owner of the tanker, which has a capacity of 33,000 tonnes of crude palm oil, said the ship had not been blocked, but Greenpeace said tugs had pushed the Rainbow Warrior aside.

"We were pushed by two big tug boats close to the MT Westama, at a distance of about 30 metres (100 ft). We tried to blockade the Westama for about an hour, but finally the ship escaped," Bustar Maitar, Greenpeace Indonesia Forest Campaigner, said.

Maitar, speaking from the ship by telephone, said the Rainbow Warrior would remain to try and prevent palm oil loading by other tankers.

The head of the Jakarta office of Permata Hijau Sawit Group, the ship's owner, said that the ship had departed at 8 a.m. after being delayed from its scheduled departure the night before.

"There was no blockade at all by Greenpeace, when the ship departed after completing its loading the Greenpeace ship moved by itself," said Hendra G., the official, who blamed the late departure on tidal conditions.

The protest came ahead of an industry meeting on sustainable palm oil in the Malaysian capital Kuala Lumpur next week and less than three weeks before a U.N. climate change meeting on the Indonesian island of Bali.

Environmental groups have blamed palm oil companies for driving the destruction of Indonesia's forests and peatlands, boostin the emission of greenhouse gases.

Sue Connor, Greenpeace international forest campaigner, said that the Rainbow Warrior would hold its position for the moment and the group was assessing its next move.

"We've certainly raised the issue of the problem with palm oil being the leading cause of deforestation in Indonesia," she said by telephone from the ship.

The group has asked Indonesia, which is set to become the world's top palm oil producer this year, to issue a moratorium on conversion of forests and peatlands into palm oil plantations.

(Reporting by Telly Nathalia and Yayat Supriatna; Writing by by Ed Davies; Editing by David Fox)

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