The number of illegal immigrants from Pakistan nearly quadrupled from 213 in 2006 to 779 last year, overtaking the Vietnamese - at 581 in 2007 - as the biggest single source of illegal foreign immigrants.

And while only one Nepali was arrested in 2006, 177 were caught last year. Other major increases in arrests were of people from Bangladesh and India, at 13 and 30 times respectively.

Mr Lee said there was no single factor in the rise in illegal South Asian arrivals, although tighter visa policies and political instability in these countries might help to explain it, along with employment opportunities in Hong Kong.

The security chief said there was also an increasing trend for illegal immigrants to seek to stay in Hong Kong by claiming protection under the UN Convention Against Torture.

'There are some illegal South Asian immigrants working in Hong Kong and they might have taught others how to use the convention to stay in the city,' he said.

Hong Kong officials had to assess each case to ensure immigrants would not face torture if they were repatriated. As this took a long time, the cases were accumulating.

'The best solution to illegal immigration is always immediate repatriation. But this is difficult in these cases,' Mr Lee said.

To stem the flow, he said, security officials had stepped up border patrols and surveillance as well as intelligence exchanges and co-operation with mainland authorities.

However, he said the mainland was facing the same problem as Hong Kong. It was estimated that Guangzhou alone already had tens of thousands of illegal workers.

'Some of these workers might be persuaded to go to Hong Kong, where it is a lot easier to find jobs and there are South Asian residents who can take care of them, too,' he said.

Lawbreakers

In 2006, Vietnamese formed the biggest single group of illegal foreign immigrants

Pakistanis took the top slot last year with their number almost quadrupling from 213 in 2006 to 779