Mr. Ek Tha, who has worked in construction for a decade, said the materials used at the site, which he said was to be a hotel, were insufficient. “They just used metal,” he said. “It’s not enough.”

Krouch Sothea, whose wife worked at a Japanese restaurant next to the crumpled building, said that the restaurant and the housing they lived in had also been destroyed. He, his wife and their 2-year-old son escaped by crawling through their roof, Mr. Krouch Sothea said.

Since construction began on the building last fall, metal rods from the site had repeatedly fallen on his home, he said. No compensation for that damage had been provided by the Chinese owner, Mr. Krouch Sothea said.

The construction blitz in Sihanoukville has alarmed residents, who complain that little of the Chinese investment flows into local pockets. Many of the service workers at hotels, restaurants and casinos are Chinese, as are real estate agents and employees at travel agencies.

Some of the casinos under construction in Sihanoukville have been promoted as being part of China’s giant state-sponsored foreign infrastructure program, called the Belt and Road Initiative. But it is unclear how gambling establishments fit into a global investment push that is heralded by Beijing as benefiting poorer economies.