NEW DELHI, Dec 22 (Reuters) - India’s army chief will begin a week-long visit to China on Wednesday, the first in a decade, reflecting gradually warming ties between the world’s two most populous nations and Asia’s biggest nuclear powers.

General N.C. Vij’s trip is designed to build trust between the two countries’ armies along their 3,500 km (2,200 mile) disputed but peaceful border, officials and analysts said.

"This is an important visit in terms of symbolism," said Uday Bhaskar of the Institute of Defence Studies and Analysis in New Delhi. "India and China have been engaged in trying to make their relationship more stable."

The neighbours fought a brief and bitter war in 1962 over a territorial dispute, but the border has been peaceful for more than 15 years and growing trade has helped to stimulate better relations between two of Asia’s largest economies.

Vij’s trip follows a historic China visit by India’s then Prime Minister Atal Behari Vajpayee in 2003, and comes ahead of a planned visit to India by China’s Premier Wen Jiabao next year.

Last year the neighbours held their first-ever military exercises, involving a handful of naval ships off China’s eastern seaboard. Then in March, defence ministers announced a new set of proposals to strengthen ties between the armed forces.

Vij is expected to meet his Chinese counterpart as well as Defence Minister Cao Gangchuan Cao during the visit from Dec. 22 to 29, according to press reports. "It is a goodwill visit," said an Indian army spokesman, without giving details.

"It comes at a time when relations have never been as good, trade is booming and there are serious political negotiations going on about the boundary dispute," said Indian foreign affairs expert C. Raja Mohan.

"Slowly relations are being put back on track, and the military part is being upgraded and expanded."

But despite the new warmth, irritations remain.

Ties suffered a setback after India conducted nuclear tests

in May 1998, just a few weeks after China’s army chief visited New Delhi.

For its part, India’s defence ministry is concerned about China’s close relationship with its South Asian rival Pakistan, and complains that Beijing has helped Islamabad develop its own nuclear missile programme.

In its annual report last month the defence ministry also noted China’s military modernisation and a "build-up" in Tibet.

Each side claims the other occupies parts of its land at either end of their Himalayan border. But they are thought to be moving, albeit at glacial speed, towards a deal which will recognise the status quo as a permanent border.

Last year Vajpayee acknowledged for the first time in writing that Tibet was part of China. In return China has signalled it is dropping its objections to Indian sovereignty over the tiny Himalayan state of Sikkim.