Bad weather is hampering the search for a plane carrying 22 people, mainly foreign tourists, that is believed to have crashed in south-west Cambodia.
The Cambodian Air Force is trying to locate the plane
Rescue teams have been focusing their search on the dense forests of Kampot province, where witnesses say they saw the plane come down.
The plane had been flying between two popular tourist destinations when contact was lost on Monday.
Hundreds of soldiers and police are taking part in the rescue effort.
But the thick clouds and heavy rain have made visibility extremely poor.
"You can't see each other more than 40 metres away," civil aviation safety chief Keo Sivorn was quoted by Reuters news agency as saying.
The tracks through Kampot's mountainous forests are so muddy that using vehicles is almost impossible, so progress is having to be made on foot, the BBC's Guy De Launey in Phnom Penh says.
Cambodian Prime Minister Hun Sen has offered a $5,000 (£2,500) reward to anyone who can help find the crash site.
But he admitted that "our hope of finding survivors is slim".
The AN-24 plane had been making a short scheduled flight from Siem Reap to the coastal town of Sihanoukville when it disappeared from radar screens on Monday morning.
Aviation authorities have suggested that bad weather might have played a part in the crash.
Conservation workers in Kampot province say they saw the plane go down but could not give a precise location.
Thirteen of the passengers were members of a South Korean tour group who were heading for the beach after spending time at the Angkor temples.
Three Czechs and the five-member Cambodian crew were also on board.
The PMT Air service has only been running since January and tourism officials had hoped it would encourage visitors to spend longer in the country, our correspondent says.
Some 250,000 tourists visited Cambodia from South Korea last year - more than any other nationality.