A man detained by Chinese authorities 20 years ago after the Dalai Lama named him as one of the most senior figures in Tibetan Buddhism is living a normal life and does not want to be disturbed, government officials said.

Gedhun Choekyi Nyima was declared by the Nobel laureate as the reincarnation of the Panchen Lama, the second most senior figure in the Gelug school of Tibetan Buddhism, at the age of six in 1995.

The boy was then taken into custody by Chinese authorities and has not been seen since, with Tibetan authorities in exile describing him as "the world's youngest political prisoner".

Nyima "is receiving education, living normally and growing healthily. He does not want to be disturbed by anyone", the official Xinhua news agency late Sunday quoted Norbu Dondup, an official at Tibet's United Front Work Department, as saying.

The Dalai Lama's designation of him was "illegal and invalid", he said.

Beijing appointed Gyaincain Norbu as its own 11th Panchen Lama, who is now a deputy to the Chinese People's Political Consultative Conference, a national debating chamber, but many Tibetans do not recognise him.

He has made numerous tightly scripted public appearances since he turned 18, and visited Hong Kong, his first trip outside the Chinese mainland, in 2012.

The 10th Panchen Lama died in 1989 after a tumultuous relationship with China's Communist leaders which saw him lauded and later imprisoned.

Asked whether Nyima was still alive, a Chinese foreign ministry spokesman declined to respond explicitly on Monday.

"What I know is consistent with what the Tibetan official has briefed you about," Hong Lei told reporters at a regular briefing.

The Central Tibetan Administration, the government-in-exile, lambasted Beijing for detaining Nyima.

"As long as people are not given free access to meet him, we will consider him as missing and held against his will," Dicki Chhoyang, minister for information and international relations, told AFP.

"He's the youngest political prisoner in the world and we continue to see his forced absence as a symptom of Chinese repression."

- Wheel of life -

The officially atheist Communist Party reiterated its right to control the process of reincarnation in a white paper issued at the weekend to mark the 50th anniversary of the forming of the Tibet Autonomous Region, China's administrative designation for the area.

"Reincarnation is a succession system unique to Tibetan Buddhism, and is respected by the state and governments at different levels of the autonomous region, the state having issued the Measures on the Management of the Reincarnation of Living Buddhas of Tibetan Buddhism," the document said, adding the system was "proceeding well".

The method of selection includes "historical conventions like drawing lots from a golden urn", it insisted -- but the Tibetan government in exile says senior lamas should make the choice.

The issue of the selection is becoming increasingly important as the Dalai Lama ages -- he turned 80 this year -- raising the prospect of Beijing seeking to name its own successor to the exiled spiritual leader.

He has suggested in recent months that he may not be reincarnated after he dies, infuriating Chinese authorities.

The Dalai Lama fled Tibet in 1959 after an armed rebellion against Chinese rule was put down by its military.

China considers Tibet to be part of its sovereign territory which it claims is backed by a long-standing historical right, and accuses the spiritual leader of separatism.

The Buddhist monk denies aspirations for independence and says he seeks "a high degree of autonomy" for the region.

He has called for limits to state-backed Han Chinese migration to Tibet, a stance which the white paper said was "tantamount to an ethnic cleansing of the plateau".