JAKARTA (Reuters) - Greenpeace has blocked a tanker carrying more than 30,000 tonnes of palm oil from leaving an Indonesian port to protest against forest destruction blamed on plantations, the environmental group said on Thursday.

The protest came less than three weeks before a U.N. climate change meeting on the resort island of Bali, where delegates from 189 countries will debate ways to slow down global warming, including the impact of dwindling tropical rainforests.

The group's Rainbow Warrior ship dropped anchor next to the MT Westama, which was set to leave for India from Dumai in Sumatra island, one of the Southeast Asian nation's main ports handling palm oil.

"We will block this as long as we can. We want the government to immediately issue a moratorium on conversion of forests and peatlands into palm oil plantations," said Bustar Maitar, Greenpeace Indonesia Forest Campaigner, who spoke by telephone from onboard the ship.

Environment groups have blamed palm oil companies for driving the destruction of Indonesia's forests and peatlands.

In a recent report, Greenpeace said that clearing forests that often grow on the country's thick carbon-storing peatlands released more than a billion tonnes of greenhouse gases a year.

The report said the surging demand for palm oil in food, cosmetics and fuel was putting pressure on a ticking "climate bomb."

The palm oil on the MT Westama belongs to Permata Hijau Sawit, one of the largest exporters of palm oil, the group said in a statement.

Greenpeace communications officer Tiy Chung said they had not been told to leave by police or been contacted by the company.

"The police came and spoke to our captain. But they left soon after," Chung said by telephone from onboard the ship.

"They were very polite."

(Reporting by Adhityani Arga; Editing by Ed Davies and Bill Tarrant)