BEIJING, June 28 (AFP) - Chinese authorities have heightened restrictions on travel to Tibet as a security measure during the period of the Hong Kong handover, the Tibet Information Network (TIN) reported Saturday.

At least 50 foreign tourists with valid visas have been turned back from the land border between Nepal and Tibet this month because they were not part of official tour groups, the London-based organisation quoted Nepalese border officials as saying.

Tibetan exiles, normally encouraged to visit Tibet as "overseas Chinese," face the stiffest restrictions, with Chinese embassies in India and Switzerland refusing to issue them travel documents until late July or August, TIN said.

China Southwest Airlines has also required foreign residents of the Himalayan region to present extra credentials an official letter of invitation from their employer in Tibet before issuing tickets in Nepal during the past three weeks, it said.

Beijing is tightening security nationwide to prevent any incidents that might mar Hong Kong's July 1 handover to Chinese sovereignty, seen as a historic moment for the country.