China has stopped issuing travel documents to foreigners seeking to visit occupied Tibet, according to local tour operators, another indicator of the government’s skittishness over the coming anniversary of the Communist victory in 1949.

The ban on new permits, which took effect on Monday, will last at least three weeks, travel agents say.

This is the third time foreign travel to occupied Tibet has been halted since March 2008, when rioting killed at least 22 people in Lhasa, the Tibetan capital, and left part of its central business district in flames.

Tour operators who arrange the paperwork said the new regulations were issued Sunday by the region’s tourism bureau without explanation. They said that foreigners already holding permits would be allowed to travel to occupied Tibet but with restrictions, including requirements that they travel only with guides and stay in government-approved hotels. Tour operators said they were told that the ban on new permits would remain in effect until at least Oct. 8.

Yong Hong, deputy sales manager at Xigaze China International Travel Service in Lhasa, said the new rules were unexpected and not particularly welcome. “It was a sudden thing, but this year is unusual,” he said, referring to the Oct. 1 National Day celebrations marking the founding of the People’s Republic.

Tourism, which makes up nearly 20 percent of the region’s economy, was battered by the rioting last year but has more than recovered, officials say. Nearly 1.4 million tourists visited the Tibet Autonomous Region in August, a monthly record, according to figures cited by Xinhua, the state news propaganda agency.

Foreign passport holders were barred from visiting Tibet in the months after the riots and again last spring, just before the 50th anniversary of a failed revolt that led to the exile of the Dalai Lama, the Tibetan spiritual leader who has long sought greater autonomy for Tibetans living under Chinese rule.

Foreign journalists are also barred from travelling to occupied Tibet except those invited to attend a rare, tightly scripted official tour.

Security has been increased across the country in the weeks leading up to National Day events and has become especially tight in Beijing, which will be the scene of a vast military parade, a fireworks display and a speech by President Hu Jintao.