FEARS of an outbreak of violence among detainees at the Whitehead Detention Centre, Hong Kong's largest Vietnamese camp, were raised last night following the announcement that the Nei Kwu Chau centre will be closed over the next two days. A hardcore group among the 400 boat people remaining in Nei Kwu Chau were branded as trouble-makers and moved from Whitehead in 1991 because they were behind widespread violence at the centre. In a transfer operation beginning today, they will be returned to Whitehead, where relative calm has been maintained since their departure. About 19,000 Vietnamese migrants are housed in Whitehead. Aid workers have expressed concern that the transfer of the Nei Kwu Chau detainees could see a return to the violence of 1991 at Whitehead, where large-scale riots took place on a regular basis. Murders, attacks on warders and faction fights were common at Whitehead during that period. It has been learned that the closure is seen by many aid workers in the detention centres as premature and that other options could have been taken. It is also understood that other proposals for Nei Kwu Chau to remain open longer or for some of the people to be transferred to High Island Detention Centre were rejected by the Government. A lawyer representing many of the Vietnamese boat people, Pam Baker, said the move was probably aimed at encouraging those screened out as non-refugees to join the voluntary repatriation programme. ''By putting a group of bad apples back into Whitehead, some of the detainees would probably choose to leave rather than get caught up in any trouble,'' she said. The Correctional Services Department (CSD) said it did not expect any problems with the transfer, and assurances were given by the Government to the United Nations High Commissioner for Refugees (UNHCR) that the camp closure and transfer of the detaineeswould not pose any problems. UNHCR chief-of-mission Jahanshah Assadi said his staff would be doing everything possible to make the transition as smooth as possible for those being transferred. He said UNHCR staff made available with the closure of Nei Kwu Chau would be transferred to Whitehead and other centres. A CSD spokesman said the camp would be cleared by tomorrow and that the detention centre would become an annex to a neighbouring CSD drug rehabilitation centre on the island.