(Lighting strikes over Saint Joseph cathedral during a storm in Hanoi July 14, 2012. REUTERS/Carlos Barria )

(Lighting over Saint Joseph’s Catholic Cathedral during a storm in Hanoi July 14, 2012. REUTERS/Carlos Barria )

A U.N. expert expressed worry on Thursday over “serious violations” of religious freedom in Vietnam following a fact-finding mission he said was interrupted by surveillance, harassment and intimidation.

Heiner Bielefeldt, U.N. Special Rapporteur on freedom of religion or belief, said parts of his trip were hampered by interference by unidentified agents, with people he met followed or questioned and others warned or blocked from seeing him.

Religion remains under state supervision in mainly Buddhist Vietnam, which has long been accused of suppressing freedom of worship by groups and individuals with faiths not registered or recognized by the communist country’s rulers.

Bielefeldt said he had seen positive developments in terms of coexistence of faiths and “cautiously widened” space to practice religions, but that was marred by breaches of agreements to guarantee unsupervised access without threats or punishment to those he met.