SECURITY has been stepped up at Hong Kong's principal synagogue. A former Gurkha soldier stood guard outside the Ohel Leah building in Robinson Road, Mid-Levels yesterday. But Jewish leaders deny the changes are connected with a series of anti-semitic bomb attacks around the world. They say the move is part of a regular review of security procedures at the site. Stuart Cotsen, chairman of the Jewish Club of Hong Kong, declined to give details of the changes. ''Security is only security if it is effective,'' he said. ''Publicity makes it ineffective. ''Our security review is on-going and any decisions were made well before any of these recent incidents. ''We try to anticipate problems rather than react to them. We are not reacting to anything in particular in this instance.'' The guard is employed by the security firm Jardine Securicor, which also declined to give details of its operations. He was on duty yesterday as people arrived for the Jewish sabbath. At least 95 people were killed in a bomb attack in Argentina's capital Buenos Aires last month and 19 were injured in two explosions in London aimed at Jewish targets.