A TOP Marine Police officer is to visit southwest Lantau to investigate a spate of burglaries thought to have been carried out by boat-based Vietnamese gangs. But yesterday Deputy Regional Commander John Hui Chiu-yin admitted that prosecutions against the gangs were proving difficult because of the Millport Policy - which provides safe passage to Vietnamese claiming to be seeking asylum as they pass through the territory. Some police also complain that the gangs operate 'with impunity' because of an unofficial policy of non-prosecution. Security Branch officials are not keen for Vietnamese to be given prison sentences which may go beyond the handover, as China has repeatedly said it is opposed to any Vietnamese being left in the territory when it takes over. Mr Hui said the police were becoming increasingly frustrated by the regularity with which gangs slipped through their net. Sources said about 10 gangs, comprising about 100 Vietnamese, were operating from mother ships anchored in Chinese waters. They were suspected of stealing goods in Hong Kong and selling them in China, or to other vessels. Mr Hui said that when challenged the gangs claimed to have bought the goods found on their boats. 'We know what they are up to, but until we can prove the goods are stolen we cannot prosecute. 'I think some of my officers are uptight because some cases they think should have been prosecuted have not been.' Last week, one of the few successful operations resulted in charges against 10 Vietnamese suspected of burglaries on Lantau. But two weeks ago, police stopped a seven-strong gang in the Silverstrand area with electrical goods on board two of their sampans. They also had a stolen boat. The boat's owner was traced to Sai Kung, but there was still considered to be 'insufficient evidence' to prosecute. The gang was the same one which had been picked up a week earlier with electrical goods on board their sampans. On each occasion, the gang was taken to Tai A Chau and given food, water, fuel and a police escort out of Hong Kong waters. Under the Government's Millport Policy, Vietnamese claiming to be passing through Hong Kong seeking asylum are entitled to such provisions. 'When we pick them up they claim to be on their way to Japan,' a source said. 'When we pick them up a second time they say they lost their way in bad weather.'