The six included an alleged snakehead who was caught as he tried to swim back to Zhuhai , said Leung Wa-kan, head of Macau customs' marine patrol division. The other five swam ashore or were washed up on the beach. One of them was taken to hospital for examination.

Leung said they told rescuers there were 17 or 18 people on board the 7-metre speedboat, and that three of them were snakeheads.

He added that the other two snakeheads and three other passengers were believed to have swam ashore and fled before officers arrived.

"Our investigations have found they came from Zhuhai to sightsee and gamble," Leung said.

Rescuers believe the speedboat capsized after being hit by heavy waves. It is less than 1km from Macau to the nearest stretch of mainland coast.

A senior Macau police officer told local media that, in the past, mainlanders had come to the city illegally to work, but recently there had been other cases of mainlanders entering Macau illegally to gamble - since mainland authorities tightened restrictions on travel documents for the city.

Leung said more than 10 boats had been dispatched by the mainland and Macau authorities to search the area, and divers were also used.

Macau's marine bureau said mainland authorities also sent a helicopter to help the search.

Customs officers retrieved the fibreglass speedboat and it was towed ashore.

Leung said that because of bad weather, the search was suspended at about 5pm. He said it would resume this morning.

There were some 1,409 reports of illegal entries to Macau from the mainland last year, according to local media.

Leung said the numbers had been steady in the past few years.