Blantyre - Malawi President Peter Mutharika on Tuesday vowed a thorough investigation after seven people were murdered by vigilantes in a vampire scare that has driven UN staffers out of the affected region.Since mid-September, vigilantes have slaughtered seven people suspected of trying to obtain and drink human blood as part of magic rituals.The vampire rumours sweeping the country's southern region have forced authorities to impose a night-time curfew, restricting movement to 10 hours from 07:00 (05:00 GMT) until 17:00 (15:00 GMT).The UN - which is involved in food aid and agricultural assistance programmes - has pulled its workers out of the area for safety reasons.Although the attacks are not targeted at UN staff, the UN instructed all its personnel working in the affected areas to temporarily "relocate" to the commercial capital Blantyre, 90km away.Four districts in southern parts of the country have been swept by rumours of bloodsucking humans, but Mulanje, which borders Mozambique, is the epicentre of the killings.Volatile, unstableIn a report last week and obtained by AFP Tuesday, the UN said it had suspended all visits to the affected areas because the "situation is still unstable and volatile"."We have instituted an intensive investigation for us to get to the bottom of the matter," the president said in a statement. Police told AFP a seventh person was killed by an angry mob on Monday in Thyolo, the president's home district.The latest victim, a mentally-disabled man, was found loitering at night in a village and was lynched by vigilantes who suspected he was pretending to be insane, said police spokesperson Lloyd Maida.Rumours of vampires allegedly originated from Mozambique and "spread across" the borders to the Malawian districts of Mulanje and Phalombe, according to the UN. Malawi, where witchcraft is widely believed and education standards are low, is regularly dogged by rumours of "vampire" activity. The United States embassy has also temporarily withdrawn its team of Peace Corps volunteers from the districts surrounding Mulanje and has advised its citizens not to visit the affected districts.