JAKARTA - The search operation to find three missing divers - one of whom is Singaporean - entered its fourth day on Wednesday (Nov 6) in waters off Banten province's Sangiang island in the Sunda Strait.

Indonesia's search and rescue agency Basarnas has been conducting search operations since a diving mate of the missing trio alerted the authorities to the incident on Sunday afternoon. At least 12 vessels, including navy ships and police patrol boats, and two helicopters were deployed on Wednesday.

The search area is now divided into four regions, measuring a total of 1,300 square nautical miles. The fourth region - south-west of the spot where the tourist divers began their excursion - is covered by air search operations and is the biggest of the four regions, Basarnas said on Wednesday morning.

Six tourists went out diving in two groups, but the missing trio - one Singaporean and two Chinese nationals - did not return to base.

There was no sign of any of the three missing divers found as of Wednesday night, according to Basarnas.

The two groups of divers started to dive at about 1.30pm local time and only one group returned to the surface at about 2.45pm, Basarnas said.

The Singaporean was identified by Basarnas as Wang Bing Yang, while the two Chinese are Qin Xue Tao and Tian Yu. All three missing are men.

The families of the two missing Chinese divers have promised anyone who could help locate their loved ones a US$50,000 (S$67,903) reward, news website liputan6.com reported.

The Singapore Embassy in Jakarta has been in contact with the family of the Singaporean, a Ministry of Foreign Affairs spokesman said on Tuesday. The embassy has been liaising closely with the Indonesian authorities.

A consular officer from the embassy is on the ground to help and give close support, the spokesman earlier said.

Mr Zaenal Arifin, head of Basarnas' Banten province chapter, has earlier told reporters that the missing tourists are licensed, professional divers and might have visited Indonesian diving spots several times.

Waters off the Sangiang island is a popular diving spot for foreign and domestic tourists. The island is reachable within one hour by boat from the nearest pier in the main island of Java.

On Sept 8, 2018, Mr James Ade Ignatius Salaka, 32, a North Jakarta resident who did a fun-dive at a depth of between 9m and 12m in the waters off Sangiang island was drifted away from his group by an unexpected strong undercurrent. He was later found dead.