mount ruapehu
Mount Ruapehu, which provided the backdrop for scenes in The Lord Of The Rings trilogy Getty Images

Hikers trekking up a New Zealand volcano featured in The Lord Of The Rings films have been warned turn back after an increase in activity. Monitors at Mount Ruapehu, depicted as part of Mordor in director Peter Jackson's trilogy, recorded an increase in volcanic gas and a dramatic rise in the temperature of its lake − from 25C (77F) to 46C (115F).

The authorities have now raised the volcanic alert level from "moderate" to "heightened unrest". Trekkers and climbers have been warned to stay away from the Summit Hazard Zone, which is within 2km of the lake's centre.

"The increase in gas output, the high lake temperature and continued seismicity suggest that Ruapehu is at a higher level of volcanic unrest," a statement from the GNS monitoring service said. "At this time these changes at Ruapehu are considered sufficient to change the Volcanic Alert Level."

Volcanic gas measurements indicated an increase in the amount of both carbon dioxide (CO2) and sulphur dioxide (SO2) output. The lake's temperature has risen more than 20C in just two weeks.

The landscape in New Zealand's Tongariro National Park formed the backdrop for Mordor in The Lord Of The Ring films. Thousands of people trek through the park every year.

Mount Ruapehu, the largest of New Zealand's active volcanoes, is the highest point in the North Island, at 2,797m. Spectacular eruptions occurred in 1995 and 1996, with the resulting ash cloud closing nearby airports and ending the ski season early. The volcano last erupted in 2007.

mount ruapehu
A tourist photographs Mount Ruapehu as it erupts on 18 June, 1996 in Tongariro National Park on the central North Island of New Zealand Reuters