But a spokesman for the United Nations High Commissioner for Refugees in Tokyo said no Vietnamese migrants had arrived in Japan this year. That suggests the boats might have tried to reenter Hong Kong or gone to another Southeast Asian destination.

Marine officers spotted the eight-metre boat packed with people off Sham Wat heading for the Chek Lap Kok construction site at about 3 am.

The boat refused to stop when two Marine Police launches and two other police boats approached it, Superintendent Leung Wai-shing said.

'The wooden boat kept circling for about half an hour before being intercepted,' he said. 'We didn't take an aggressive approach in the chase because it was a small boat and many people were on board. We didn't want anyone get hurt.' The group aged between 17 and 38 said they left from the northern port of Haiphong on Friday, each claiming to have paid US$100 (HK$773) to get to Japan.

A Security Bureau spokesman said the high number of new arrivals would delay the emptying of the one remaining Vietnamese detention centre at High Island.

It takes four to six months for authorities in Hanoi to agree to their people's return even if they do not claim political asylum.

'It takes a long time to clear their status so they can be returned and during that time we have to keep them,' the spokesman said.

A man tried to hang himself before being put on to a forced repatriation flight to Hanoi yesterday.

The 23-year-old was taken to hospital for observation while 135 other Vietnamese, mostly men who had arrived this year, were deported on the 111th Orderly Repatriation Flight operated by the Government.