WELLINGTON, May 13 (Reuters) - Volcanic activity at New Zealand's Mount Ruapehu is increasing and an eruption could occur at any time, scientists warned on Tuesday. The volcano in central North Island, famed as a location in the "Lord of the Rings" film trilogy, last erupted on Sept. 25 2007, spitting 2 metre (6 feet) boulders distances of up to 2 kms (1.5 miles).

Ruapehu's elevated alert level has not been changed, but scientists said on Tuesday that activity within the mountain was greater, with high levels of gas spewing out, a warmer than average crater lake and ongoing volcanic tremors.

"The volcano remains in a status of unrest and the possibility of further activity remains. If further eruptions occur, they may occur without warning," Geological and Nuclear Sciences (GNS) said in a statement.

Last September's eruption injured a climber after a boulder crashed through the roof of a hut near the summit where a party of four climbers were staying.

The highest mountain in New Zealand's North Island at 2,797 metres (9,177 feet), Mt Ruapehu has one of the most active crater lakes in the world.

In March 2007 a mudflow, or "lahar" flowed down the side of the mountain after the crater lake overflowed, and large eruptions in 1995 and 1996 blanketed the surrounding area in ash.

In 1953 a lahar swept away a railway bridge at Tangiwai at the base of the mountain and 151 people were killed when an Auckland-to-Wellington train plunged into a river. (Reporting by Adrian Bathgate; Editing by Michael Perry)

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