Arbour made her request on March 27, but the Chinese authorities got back to her this week to say a visit would not be possible at the current time, spokesman Rupert Colville said. "They didn't exclude the possibility of a visit at a future date," he added.

Arbour will step down from her post in June after four years as the UN High Commissioner for Human Rights. She had already called on Beijing to "allow demonstrators to exercise their right to freedom of expression and assembly, to refrain from any excessive use of force while maintaining order," after violence erupted in the Tibetan capital Lhasa in mid-March.

Tibetan leaders in exile say the recent crackdown in the Himalayan region has left more than 150 people dead but China has denied those claims. Beijing says Tibetan "rioters" have killed 20 people.

A group of UN human rights experts, including the special rapporteur on torture Manfred Nowak, also expressed concern at the crackdown on Thursday and urged China to grant observers free and unfettered access to Tibet.