Media playback is unsupported on your device Media caption Some of the survivors swam for seven hours, reports Karishma Vaswani

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 early on Monday.

They are being treated in hospital for trauma and dehydration as a search continues for the remaining two people.

The Indonesian military has joined the search to find the two remaining tourists, a Dutch man and an Italian woman.

Speedboats and a helicopter have been deployed, but authorities say rescue efforts are hampered by bad weather and high waves.

Officials said the incident took place when the boat, sailing from Lombok island to Komodo island, hit a wave that was 3m high and crashed into a reef. It sprung a leak and sank.

Rescue officials told agencies that the second group were found together by fishermen. Some of them were in a lifeboat while others were floating in their lifejackets.

The survivors were now recovering in Sape city in Sumbawa, they said.

Image copyright AFP Image caption A group of survivors rest at a house near the Indonesian town of Bima following the sinking on Saturday

Image copyright EPA Image caption The survivors split into two groups shortly after the boat sank

Survivors said that as the boat started sinking, some got into a lifeboat which could only take up to seven passengers. Others climbed onto the roof of the boat which had not yet completely sunk.

They eventually split into two groups when it became clear that there were not enough lifeboats nor communication tools to call for help.

One group decided to swim six to seven hours to reach Sangeang Api, while the others stayed with the lifeboat.

French survivor Bertrand Homassel was in the first group. He told AFP: "We were 5km (three miles) from the coast and there were many big waves separating us from the coast. People started to panic and everyone took the decision to swim to the closest island."

After spending the night on the island by eating leaves and drinking their own urine, that group was rescued by a passing boat on Sunday.

The other survivors who stayed with the lifeboat, who were rescued on Monday, were in the water for about 40 hours.

Dutchman Jan van Ommen said people took turns to spend time in the lifeboat and float in the water with their lifejackets.

"So we had this system, and in the beginning it was not easy... but later on the system went on, and we changed and changed," Mr van Ommen told AFP.

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

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

The BBC's Alice Budisatrijo in Jakarta said the boat was on a three-day journey through some of Indonesia's most picturesque islands.

Boats are the main form of transport in some parts of Indonesia, but accidents are common because of poor safety standards.