At least four people have died after an under-construction building in Cambodia collapsed with more than 30 people on site, police said.

The seven-storey building in the coastal province of Kep collapsed on Friday. By night, 20 injured workers were rescued, provincial authorities said.

The incident came a year after 28 people were killed in another construction site collapse in Preah Sihanouk province.

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Nguon Samet, deputy police chief for Kep province, said the building apparently collapsed when cement was being poured on its top level.

Recovery operations were continuing with debris being removed to try to locate any people still trapped.

Police spokesman Chhay Kim Khoeun said the cause of the collapse, nature of the structure and ownership of the building were unknown.

"Now, we focus on rescuing people," he added.

Prime Minister Hun Sen posted on his Facebook page that he would "depart to Kep province shortly to lead the rescue team, helping to rescue the workers that got trapped under the collapsed building".

The collapse of the building under construction last June in Sihanoukville, another coastal province, killed 28 construction workers and injured 26 others, underlining concerns about the area's rapid development and inattention to safety.

The coast has been rapidly built up to cater to a booming tourist industry.

In December, a Buddhist temple collapsed while under construction in Siem Reap, home to Cambodia's famed Angkor temples, killing at least three people and injuring 13 others, including two monks.

Cambodia is undergoing a construction boom to serve growing crowds of Chinese tourists and investors.