BEIJING, May 18 (Reuters) - China barred a prominent AIDS and environmental activist and his wife from leaving the country on Friday, put them under house arrest and accused them of endangering national security, the pair said.

Plainclothes police took Hu Jia away from his home for questioning hours before he and his wife and fellow AIDS campaigner Zeng Jinyan were to board a plane bound for Hong Kong en route to nine European countries, Zeng said.

Zeng, who is three months pregnant, was recently named by Time magazine as one of the world's 100 most influential people.

Hu was released after about five hours of questioning and told that he and his wife were suspected of "endangering national security" and barred from leaving the country, Hu said, adding that they would be under house arrest for an indefinite period.

Hu's activism has set him up on a collision course with the Communist Party, which has stepped up curbs on NGOs, the media, the Internet, lawyers, academics and civil rights campaigners to maintain its grip on power.

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