"Maintenance costs (for the railway) could be pushed up because of the unusual climatic conditions," he said. "Safety of passengers on the new railway and a new highway in Tibet could be affected by global warming."

Experts have voiced fears that parts of the track could become unstable, triggering derailments if warm weather melted frozen ground under the railway route.

The 1,142-kilometre (710-mile) track climbs a peak of 5,072 metres (16,737 feet) above sea level, making it the highest railway in the world.

The chairman of the Tibetan Autonomous region, here to attend the current session of China's parliament, denied that the railway was unsafe but admitted that a train had derailed recently.

"There was one derailing incident recently, but there were no casualties," Qiangba Puncog told a press conference on Thursday without giving further details.

Last year was China's hottest in half a century and more than a third of climate observation stations on the Tibet plateau registered all-time high temperatures, according to the China Meteorological Bureau.

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