THE military-style operation at Whitehead Detention Centre was a setback to efforts to persuade Vietnamese boat people to return home voluntarily, the United Nations High Commissioner for Refugees (UNHCR) said yesterday. There was a climate of ''mistrust'' after the operation to remove 1,500 boat people from the centre to High Island on April 7, chief of mission Jahanshah Assadi told legislators on the security panel. A total of 1,500 people were transferred from Section 7 in the all-day operation which involved 1,250 Correctional Services Department and police officers in riot gear who fired an unprecedented 557 tear-gas canisters. ''The exacerbated climate of mistrust has got worse. This will not help us in our work,'' Mr Assadi said. ''This is a setback to our efforts to promote a durable solution for the [Vietnamese] population here in Hong Kong,'' he said. Mr Assadi denied that the UNHCR had supported the Government's operation. He said instead he was shocked and taken aback about the number of injuries. The Government at first said only one person had been hurt, but later admitted more than 200 had reported injuries. An independent inquiry into the raid is under way. Mr Assadi said he had been told few details when he was informed of the operation 12 hours before it started. ''If someone had told me beforehand that tear-gas would be used and there would be 200 or so injuries, obviously we would have vigorously protested [against] it,'' he said. But he said the operation came as ''no surprise'' to him as things had been tense in Whitehead in recent months. Boat people there had demonstrated and staged hunger strikes in the past three months to protest against repatriation. ''This is precisely why we ask the Vietnamese to cut this out. Don't give the Government an opportunity to crack down because no one will win in that situation,'' he said. Mr Assadi said the UNHCR would conduct its own inquiry. Secretary for Security Alistair Asprey told legislators the raid was necessary because the Government had received intelligence reports that there had been ''significant risks of confrontation and of injury both to Vietnamese and Correctional Services Department officials''. The reports said there had been intimidation and manufacturing of weapons among the boat people who conducted ''peaceful'' protests. ''They need to resist repatriation continuing at this stage,'' he said. ''But we do not think we can change our policy on repatriation. So, there is a fundamental conflict here between the Vietnamese and the Government's policy.'' Mr Asprey said only 300 officers were deployed to clear Section 7, with others cordoning off other sections to prevent ''concerted opposition'' from the whole Whitehead centre. ''We deployed 1,200 people, not to deal with the 1,500 people in Section 7 but to deal with the potential 15,000 people in the whole of Whitehead,'' he said. ''In Section 7, we deployed fewer than 300.'' Mr Asprey also supported the use of tear-gas. ''I was aware if there was resistance, tear-gas was likely to be used,'' he said. ''The use of tear-gas in such a situation is considered the most effective measure for securing control with minimum force. ''It reduces the requirement for the deployment of force and physical contacts''.