United Nations human rights investigators are calling on China to disclose the fate of more than 300 Tibetan Buddhist monks they say have been subjected to enforced disappearances after being taken into custody at a monastery in Sichuan province earlier this year.

The U.N. Working Group on Enforced or Involuntary Disappearances says the monks at the Kirti Monastery were arrested by Chinese riot police April 21 and hauled away in 10 military trucks to unknown destinations. The monks had been conducting weeks of protests against China's policies toward Tibet.

The group's statement issued Wednesday called for full investigations into what rights activists say is China's policy of enforced disappearances. It calls such detentions a “terrible practice” that can not be justified under any circumstances.

There was no immediate Chinese response to the U.N. statement.

Video obtained and aired in April by VOA showed Chinese security forces patrolling near the monastery. It also showed images taken weeks earlier of a young monk hours before he died, covered with burns and apparently in shock after setting himself on fire to protest the anniversary of a deadly 2008 Chinese crackdown in Tibet.

Days before the video aired, the U.S. State Department said China's use of force at the monastery was inconsistent with freedom of religion and human rights. The Chinese Foreign Ministry called the U.S. statement “irresponsible.”

Many Tibetans are angry about Chinese rule, and what their supporters say are Beijing's efforts to suppress Tibetan traditions and religion.

China has repeatedly denied such discrimination, and says its funding in Tibet has significantly improved living standards in the Himalayan territory.