Cambodia deployed soldiers, police and divers to scour an island popular with backpackers after a British tourist went missing there four days ago, an official told AFP on Monday.
Nearly 200 members of the army, navy and police have fanned out across Koh Rong in southern Cambodia in an attempt to find Amelia Bambridge, who was last seen at a beach party around 3:30 am on October 24.
"Divers are searching in the sea around Koh Rong while the others are scanning the jungle," said Kheang Phearun, a spokesman for the Preah Sihanouk provincial administration.
"We have not yet found the missing British woman."
British media reports said the 21-year-old had befriended other tourists, but the alarm was raised after she failed to check out of her hostel.
The mystery deepened after authorities said they found her bag and phone where she was last seen, a late-night hangout called Police Beach.
Bambridge's family have arrived in Cambodia and headed to the closest city, Sihanoukville, on Sunday night, Phearun added.
Located in the Gulf of Thailand, Koh Rong is a two-hour boat ride from the coast and draws budget travellers with its cheap guesthouses, beachside bars and idyllic beaches.
But it has also undergone development in recent years in keeping with a construction and casino boom in nearby Sihanoukville.
Cambodia has long been a stop for tourists travelling around Southeast Asia.
Though generally safe, crimes involving foreigners have grabbed headlines in the past.
Last week a Cambodian court charged three men with gang-raping a French tourist in the coastal province of Kampot after offering her a ride in their car.
In 2013, Kampot town was rocked by the discovery of a mutilated body of a 25-year-old Frenchwoman floating in a river.
A Belgian was charged with the alleged rape and murder of the woman, but released on bail due to a lack of evidence.