The family members of three missing divers, including a Singaporean, yesterday joined Indonesian rescue vessels as the search was expanded to the waters of neighbouring Lampung province in Sumatra.

"As of 3pm local time (4pm Singapore time), there was no sign of the missing divers," said Mr Muhammad Zaenal Arifin, a senior official at Indonesia's search-and-rescue agency Basarnas.

Six tourists went diving in two groups on Sunday. They started their dive at about 1.30pm local time but only one group returned to the surface at about 2.45pm.

The missing trio - a Singaporean and two Chinese nationals - did not return to base on Sangiang Island in Banten province, near Jakarta.

Basarnas believes one group went north of Sangiang, while the missing trio headed south. They might have encountered strong undercurrents which flow from the sea north of Java and the Strait of Malacca into waters between Sumatra and Java islands.

On Thursday, Basarnas deployed 19 divers, up from 13 on Wednesday, and is sending more vessels to search the sea surface. Three helicopters were also deployed, from two previously.

"Families of the victims have joined the search boats. The relatives of the Singaporean victim - his child and in-laws - are on a search boat in waters off the Anak Krakatoa volcano," Mr Zaenal, who heads Basarnas' Banten chapter, told The Straits Times.

"They were on a search helicopter yesterday (Thursday). We are facilitating victims' family members who want to join the search."

A Basarnas search boat with the Chinese victims' relatives reached Sebesi Island, in Lampung province, yesterday, looking for any sign of the missing divers and interviewing residents, Mr Zaenal said.

Search personnel interview villagers on islands they visit as a matter of course, in case they have useful information. Basarnas has also notified fishermen in the region about the missing divers.

Sangiang and Sebesi are in the Sunda Strait, between Java and Sumatra. The area has strong undercurrents which have swept away divers in the past.

On Sept 8 last year, Mr James Ade Ignatius Salaka, 32, a North Jakarta resident diving off Sangiang Island, was separated from his group by an unexpectedly strong undercurrent. He was later found dead.