Eighteen people were killed and at least 24 other people were injured when an under-construction building in Cambodia collapsed on Saturday, an official said, as rescuers struggled to reach missing workers feared trapped under a mountain of twisted steel and rubble.

The accident happened around 4:00 am on Saturday in the coastal town of Sihanoukville, the largest in Preah Sihanouk province. The building was nearly 80 percent complete when it collapsed.

At least 1,000 rescue workers, soldiers, police, medics and construction experts have been deployed to the site. Some wore hard hats and oxygen tanks as they searched alongside several construction diggers, while the relatives of the injured waited at nearby hospitals for news.

A statement issued on Sunday by local officials said 40% of the debris from the site had been cleared. It was not clear how many more people were missing, but local officials said earlier there would normally have been about 50 workers on the building site at the time.

"The steel structure has collapsed on itself and we don't dare move it," the spokesman Oar Saroeun told Reuters on Saturday.

"We can only wait and listen for any signs of life..."

"We are afraid more of it will collapse on them.... We will work through the night to remove the steel."

Cambodian police have detained four people, including the owner of the building, the head of the construction firm, the contractor and the landowner for questioning in relation to the accident, according to a statement from the province.

Cambodian officials said they have launched an investigation.

Cambodia's prime minister expressed his condolences to victims' families and said they would be given 10,000 U.S. dollars each.

"The tragedy of the building collapse in Preah Sihanouk province is painful... for our nation, especially the families of those who lost (their lives)," Hun Sen said.