A project to seal 180 kilometers of a major trade route on the China-Nepal border could take up to three years as workers deal with the difficult terrain in the region.

Workers have already sealed the 620-kilometer China side of the Sino-Nepal Highway from Lhasa to Gangga, but the final stretch between Gangga and Zham looks set to be the most difficult.

To asphalt the last 180 kilometers will cost 800 million yuan (about US$100 million) and it will be carried out in two stages, said Wang Riquan, political instructor of the No.2 transport detachment of the Armed Police, who is in charge of the roadworks.

The first stage would be Gangga to Nyalam, followed by the second stage of about 30 kilometers from Nyalam to Zham.

The improvements to the Nyalam-Zham section, which had been built into the mountains and bordered a deep ravine, would be especially difficult, said Wang.

"Only single-way traffic is possible on the Nyalam-Zham section, the end of the China section, " said Wang.

The mountainous terrain, high altitude and severe weather conditions that comprised rains in summer and heavy snow and avalanche risks in winter would all contribute to the difficulties, said Wang.

The earth and sand highway connecting Lhasa, capital of China's Tibet Autonomous Region, and Katmandu, capital of Nepal, was begun in 1958, and was put into service in 1965.

Sino-Nepal Highway has a total length of more than 900 km, of which 800 km are in China and it connects with the National Highway 318.

The unsealed roadbed was unable to sustain growing economic activity and trade between China and Nepal.

Since the opening of the Qinghai-Tibet Railway last year and the improved Sino-Nepal highway, trade between China and Nepal has risen rapidly.

Zham port, the only state-level land port in Tibet for foreign trade, recorded 42,000 tons of freight for import and export, with a value of 120 million U.S. dollars, in the first half, up 62 percent and 47 percent from the same period last year, according to the Zham Port administration.

About 7,000 tourists cross the border at Zham each day.