A U.S.-based rights group says Chinese authorities have detained hundreds of Tibetans who recently returned from India, where they received religious instruction from their exiled spiritual leader, the Dalai Lama.

The New York-based Human Rights Watch said Friday it believes the detentions are the first since the 1970s in which ordinary Tibetans with valid visas and passports have been rounded up in such large numbers and forced to undergo “re-education.”

China has not offered details of the crackdown. But the official Xinhua news agency on Friday quoted Foreign Ministry spokesman Liu Weimin as saying measures to strengthen “social management” in China's Tibetan areas are “absolutely necessary” to combat “separatist activities.” Liu said he was not aware of the detentions, which come ahead of Tibetan New Year's celebrations February 22.

The HRW statement called the crackdown “an abuse of Chinese and international laws,” and called for the immediate release of the detainees.

The rights group also said the detentions — nearly 1,000 kilometers west of flashpoint Tibetan areas in Sichuan province — show that the crackdown has spread to cover the entire Tibetan plateau.

More than 20 Tibetan Buddhist monks, nuns and their supporters in Sichuan have set themselves on fire to protest Chinese rule since March 2011, when a young monk set himself ablaze while calling for the return of the Dalai Lama to his homeland.