Indonesian rescuers searching for three missing divers, including a Singaporean - who might have been carried away by undercurrents - expanded their search yesterday to an area covering 1,400 nautical sq miles between Java and Sumatra islands.

The Sunda Strait, the narrow waterway between Java and Sumatra, has strong undercurrents which have swept away divers in the past.

"We have entered the fifth day. We are taking into consideration an exhaustion factor. We are adjusting down the duration, the time (our) divers spend below and increasing rotations (of divers deployed)," Mr Muhammad Zaenal Arifin, a senior official at Indonesia's search-and-rescue agency (Basarnas), told The Straits Times via telephone.

"As we search for victims, we must ensure our search personnel remain in good physical shape."

On Wednesday, the search area covered 900 nautical sq miles.

Six tourists went diving last Sunday in two groups, but the missing trio - the Singaporean and two Chinese nationals - did not return to base on Sangiang island in Banten province, near Jakarta.

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

Yesterday, 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.

As of noon local time (1pm Singapore time) yesterday, three sorties of divers had been sent down. The ideal timing for search operations is between 6am and 8am when sea currents are weakest. After that, the crews must apply extra caution, said Mr Zaenal, who heads Basarnas' Banten chapter.

He believes one diving group went north of Sangiang and that the missing trio headed towards the south of the island. 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.

"Undercurrents there can be unpredictable and change regularly depending on the changing weather. Each island there sees different sea current paths around it," Mr Zaenal explained, referring to islands in the Sunda Strait.

The strait has seen a number of accidents, including high tides capsizing boats and divers getting carried away, according to Mr Zaenal.

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

The waters off Sangiang island are a popular diving spot for foreign and domestic tourists. It takes about an hour to reach the island by boat from the nearest pier on Java island.

"There are a number of islands there and of course, there is Krakatau volcano. It is not recommended to dive there without having a local guide," Mr Zaenal said, adding that guides are able to suggest the right timings and recommend spots that are safe based on weather conditions.

Basarnas will continue its search operations until tomorrow - the last day of a standard seven-day search period - and will then evaluate whether to continue. "It will depend on whether the chances of finding the missing divers are very slim or even none," Mr Zaenal said.

He added that the victims' families could request that operations go on, but certain conditions apply.