Two foreign tourists are still missing after a boat sank off the coast of eastern Indonesia on Saturday.

Twenty foreign tourists, an Indonesian guide and four crew were on board when the boat foundered near Sangeang Api, a volcanic island off Sumbawa.

Ten people were rescued initially, and another 13, including all the crew, were found alive on Sunday.

They are being treated in hospital, but are not in serious condition, as a search continues for the remaining two.

"The incident took place... when the boat hit a reef and sank," the Agence France-Presse news agency was told by an official, Suryaman, who goes by one name.

"Fishermen managed to rescue five of them alive later at night on Saturday, and five other foreigners were rescued by a sailing boat."

Some of those on board managed to swim to a nearby island, one survivor said.

"We were 5km (three miles) from the coast, there were many big waves separating us from the coast," said Frenchman Bertrand Homassel.

"People started to panic. Everyone took the decision to swim to the closest island, 5km away, where there was an erupting volcano."

After spending the night on the island, the group were rescued by a passing boat on Sunday.

The other tourists came from the UK, Germany, Spain, the Netherlands and New Zealand.

The UK Foreign Office says two British nationals were hospitalised following the incident.

Accidents at sea have frequently occurred when migrant boats navigate through the archipelago, but accidents involving tourist boats are rare.

Source: BBC