The seven-storey building in the beach town of Sihanoukville, owned by a Chinese company, collapsed with workers inside.
“We have pulled out a body and we see two more bodies still stuck between debris,” Yun Min, the governor of Preah Sihanouk province, told Agence France-Presse.
More than a dozen others were injured, officials said, with at least two in hospital in a serious condition.
The building belonged to a Chinese national who rented the land from a Cambodian owner.
Thul Phorsda, police chief of Sihanoukville City, confirmed the death toll and said a Chinese woman was being “detained for questioning” over the fatal accident, but declined to give further details.
With the rescue attempt stretching into the afternoon, authorities remained unable to say how many people were working at the time in a country where informal labor is common.
Governor Yun Min said around 50 workers would normally have been on the site at the time.
“We have rescued 20 so far… we still can’t say exactly how many are trapped in the debris,” he said, adding some may not have been at work at the time of the accident.
In a Facebook post, Information Minister Khieu Kanharith put the number buried at “more than 30,” while Ministry of Labor suggested it could be fewer than 10.
“Authorities are working to save them carefully,” the statement added.
All of the dead were Cambodian, including two workers and a translator.
Rescue workers in hard hats heaved chunks of broken cement from the flattened building, looking for anyone buried underneath, while the diggers shunted twisted metal to the side.
Medical workers attended to a shirtless injured man, as concerned crowds built up around the site.
Sihanoukville was once a sleepy fishing community before being claimed first by Western backpackers, and then wealthy Russians.
Chinese investment has flooded in in recent years, spurring a construction boom in a resort town known for its casinos which pull in mainland tourists.
There are around 50 Chinese-owned casinos and dozens of hotel complexes under construction.
Between 2016 and 2018, $1 billion was invested by Chinese government and private businesses in the Preah Sihanouk province, according to official statistics.
Cambodia, one of Southeast Asia’s poorest countries, has notoriously lax safety laws and labor protections. Accidents are common at its building sites.
AFP / BY AGENCE FRANCE-PRESSE