China started on Sunday to build an extension to the world's highest rail link, the Qinghai-Tibet Railway, to Xigaze, the second largest city in  southwest China'sTibet Autonomous Region.

The 253-km extension from Tibet's capital Lhasa to Xigaze will pass through five counties and over the 90-km long Yarlung Zangbo Grand Canyon.

The construction will take about four years.

It is the first extension of the Qinghai-Tibet Railway that opened in July 2006.

The Lhasa-Xigaze section of the railway, with a budget of 13.3 billion yuan (1.95 billion U.S. dollars), has a designed transport capacity of 8.3 million tonnes annually.

Minister of Railways Liu Zhijun said the extension is a key project in China's long and medium-term railway network expansion and will speed up Tibet's social and economic development.

It will play a vital role in boosting tourism and promoting the rational use of resources along the line, he said.

Xigaze, with a history of more than 600 years, is Tibet's second largest city and the traditional seat of the Panchen Lamas.

Xigaze City is the administrative center of the Tibetan prefecture of the same name, a 182,000-square-km area that neighbors India, Nepal and Bhutan. It is also famous for Qomolangma (known as Mount Everest in the West), which rises up from it.