Vietnam has sent a female activist to a drug rehabilitation center for two years without trial for taking part in anti-China protests.

A lawyer for Vietnamese activist Bui Thi Minh Hang says she was detained on November 27 after joining a small protest in southern Ho Chi Minh City.

He says the next day she was transferred without trial to Thanh Ha drug rehabilitation center in northern Vinh Phuc province, near Hanoi.

The lawyer says he has lodged a complaint with local authorities but has received no reply.

Bui Thi Minh Hang is a land rights activist who recently emerged as a prominent critic of the Chinese government. She participated in protests against Chinese territorial claims on the disputed Spratly and Paracel islands that took place in Hanoi and Ho Chi Minh City between June and August.

The United States and the New York-based rights group Human Rights Watch have called on the Vietnamese government to immediately free the activist.

The U.S. embassy in Hanoi said in a statement Thursday that “no person should be imprisoned for exercising their freedoms of expression or peaceful assembly, or any internationally recognized human right.”

Human Rights Watch says the Vietnamese government has a long history of arbitrarily arresting and detaining people who speak out on what are deemed sensitive foreign policy issues.