GALLE, Sri Lanka — Bodies washed up on tropical beaches and piled up in hospitals Monday, raising fears of disease across a 10-nation arc of destruction left by a monster earthquake and walls of water that killed more than 23,000 people — a toll expected to rise with thousands still missing and millions left homeless.

Humanitarian agencies began what the United Nations said would become the biggest relief effort the world has ever seen.

The disaster could be the costliest in history as well, with "many billions of dollars" of damage, said U.N. Undersecretary Jan Egeland, who is in charge of emergency relief coordination. Hundreds of thousands have lost everything, and millions face a hazardous future because of polluted drinking water, a lack of sanitation and no health services, he said.

More than 12,500 people died in Sri Lanka, nearly 5,000 in Indonesia and 4,991 in India. The International Red Cross, which reported 23,700 deaths, said it was concerned that diseases like malaria and cholera could add to the toll.

Salt Lake City resident Ben Merkley said his son and daughter-in-law were on a small atoll in the Maldive Islands when the killer waves hit. Fortunately, Dan Merkley and his wife, Marshawn, escaped injury because their island happened to be sheltered by a larger island that took the brunt of the hit.

"They could see the water rising, and it came to the top of the beach," Ben Merkley said, after speaking Sunday by cell phone with his son, an actor who portrayed park ranger Tartan Jones in this summer's Mormon-theme movie, "Baptist At Our Barbecue."

Considering that the couple had considered Sri Lanka as a vacation destination and had also spent the previous day on Male, which "was inundated with water," Ben said he was grateful the couple was unharmed.

"I was astounded," he said. "It all missed them by only a hair's breadth."

Late Monday, Indonesian Vice President Yusuf Kalla was quoted as saying he believed the toll in the country could be as high as 25,000, that would be 20,000 more deaths than confirmed there so far and push the overall death toll to 42,000.

"We don't have confirmed data, but I think between 21,000 and 25,000 people (have died), he said, according to the Antara state news agency.

Dazed tourists evacuated the popular island resorts of southern Thailand, where the Thai-American grandson of revered King Bhumibol Adulyadej was listed as one of more than 900 people dead. Scores more died in Malaysia, Myanmar, Bangladesh, the Maldives. The waves raced 2,800 miles across the Indian Ocean to Africa, killing hundreds of people in Somalia and three in the Seychelles.

Eight Americans were among the dead, and U.S. embassies in the region were trying to track down hundreds more who were unaccounted for.