The incident happened early in the morning in Preah Sihanouk province. The Preah Sihanouk Province's Administration said that a woman was among those found dead under the debris and 21 people were injured.
The building was owned by a Chinese company and search for survivors was ongoing. Four people were arrested over the collapse, including the Chinese building owner, the head of the construction firm and the contractor, the BBC reported.
A Cambodian landowner was also taken in for questioning.
Preah Sihanouk Provincial Governor Yun Min told Xinhua news agency that between 40 and 50 workers were sleeping in the building when it collapsed.
The Ministry of Labour said in a statement that the injured people were rushed to the Preah Sihanouk Provincial Hospital and the National Social Security Fund would cover for their medical expenses.
The Chinese Embassy in Cambodia said that local Chinese companies had joined the rescue, sending one large excavator, three earth-moving vehicles and 60 people to help.
Seng Loth, a spokesman for Cambodia's Ministry of Land Management, Urban Planning and Construction, said the building was constructed without permission.
"It was an unlicensed project and the provincial authorities had warned the developer twice, but they ignored the warnings," he said.
( With inputs from IANS )