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Hundreds of family members waiting in desperation at a small port on Indonesia's Lake Toba for news of missing relatives performed mass prayers Thursday as the search for more than 190 people unaccounted for after a ferry sinking continued for a fourth day.

Only 18 people have been rescued and four confirmed dead since the overcrowded ferry sank early Monday evening in waters that officials say are up to 500 meters (1,640 feet) deep.

The disaster, likely Indonesia's worst sinking in more than a decade, has prompted Indonesia's President Joko "Jokowi" Widodo to call for an overhaul of safety standards for passenger boats. Ferries are an important means of transportation in the nation of more than 17,000 islands, which cover an area that would stretch from New York to London.

Cellphone video taken from another ferry that attempted to rescue people after the sinking has spread widely online and on television. The video shows dozens of people struggling in rough waters and crying for help while several of them try to swim for an orange lifesaver apparently thrown from the ferry.

Maruddin Siagian, waiting with other family members for news of his younger brother, said they're haunted by those images.

"We hope the government will never stop the search, never stop, until all the victims are found," Siagian said.

"We're tormented waiting for news about him, without any certainty like this. Especially our mother," he said. "But we're determined to keep waiting until the body of our brother is found. For us, his body is precious, we want to bury him properly for his soul to be peaceful, and so we are too."

The 1,145-square kilometer (440-square mile) Lake Toba, formed from the caldera of an ancient super volcano, is a popular sightseeing destination on the island of Sumatra and one of 10 stunning natural attractions in Indonesia that the government aims to develop as magnets for international and local tourists.

At a press conference on Wednesday evening, Jokowi said he'd instructed the search and rescue team involving divers and an underwater drone to find victims quickly and ordered the Ministry of Transport to review safety standards.

"This tragedy is a lesson for all of us to always be cautious and vigilant," he said. "The government will provide compensation to the families of the victims who died and guarantee the cost of care for those who need treatment."

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Karmini reported from Jakarta, Indonesia.