Macau's marine department said that four people had managed to swim to safety but the rest were missing.There were at least 10 people on board and the boat was "suspected to be linked with illegal immigration", a statement from Macau's government information service said.

The boat sank off the luxury Grand Coloane Resort at 0550 am (2150 GMT Thursday). "Customs has already found the boat and dragged it to shore," the statement said. "The rescue operation is ongoing but is difficult due to strong currents."

The four who had swum to safety were two men and two women who were then arrested, according to the Apple Daily newspaper in nearby Hong Kong.

Some local media reports said there were 19 people on the boat, including three crew members.China's state news agency Xinhua also reported there had been 19 on board and said 15 were missing.

The waters of casino-rich Macau are busy with craft plying the route to Hong Kong, which is an hour away by high-speed ferry.

Last June, 57 passengers were injured when aferry coming from Hong Kong crashed into a seawall off Macau