Tourists from New Zealand, Spain, France, Germany and the Netherlands are thought to be among those rescued after a boat reportedly hit a reef in bad weather during the three-day journey in Indonesia.
While two Britons were among the survivors, the fate of the other 15 people on board, thought to be of various nationalities and reportedly including four Indonesian crew members and Indonesian guide, is not yet known.
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The group paid tribute in the Mamayev Kurgan memorial park.
A nine-year-old schoolgirl was among the victims around the city of Osaka.
Supporters are still arriving in the city.