Religious services and memorial ceremonies have been held in Indonesia and Thailand to mark the 15th anniversary of the Indian Ocean tsunami, one of modern history's worst natural disasters.

Thousands of people knelt in prayer in Aceh province in Indonesia, the country which suffered the most casualties with more than 170,000 people killed or missing and assumed dead.

The Dec. 26, 2004, tsunami was triggered by a magnitude 9.1 earthquake off Sumatra island and about 230,000 people died in a dozen countries as far away as East Africa.

Tens of thousands of the missing have never been accounted for.

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