Amelia Bambridge has been missing since last Wednesday. Cambodia's Maj. Gen. Chuon Narin, police chief of coastal Preah Sihanouk province, said a search is taking place for Bambridge who has not been seen since she attended a beach party on Koh Rong island in southwestern Cambodia attended by tourists of various nationalities. Picture: Lucie Blackman Trust/Family via AP

Phnom Penh - Cambodian police said they had questioned at least 20 people but made no arrests as they continued the search for a 21-year-old British woman who went missing last week, as her brother urged renewed efforts to try to find her.

Divers, land-based teams and police drones have been enlisted in the search for Amelia Bambridge, who was last seen on Thursday leaving for a beach party on the island of Koh Rong, which is popular with backpackers.

The next morning, a bag containing her watch and mobile phone - which showed photos posted online until 3:23 a.m. - was found on a rock near the water's edge. Her movements thereafter are not known.

"We are very worried that's why we are using all forces to search for her," Koh Rong town governor Nuon Bunthol said.

Cambodia soldiers offer prayers in hopes of finding British national Amelia Bambridge on Koh Rong island off southwestern Cambodia. Around 150 people are taking part in the search for Bambridge. Picture: Heng Sinith/AP

Preah Sihanouk province Police Chief Chuon Narin said at least 20 people had been questioned on whether they met Bambridge the night of the party.

Sihanoukville Province Authority Police officers show a map of Koh Rong where British national Amelia Bambridge went missing. Picture: Sihanoukville province Authority Police via AP

"We are questioning as many people as possible and we haven't detained anyone," he said, adding that he feared she might have drowned.

Cambodia soldiers leap rocks after offering prayers in hopes of finding British national Amelia Bambridge on the shore of Koh Rong island off southwestern Cambodia. Picture: Heng Sinith/AP

Her brother Harry Bambridge wrote in a Facebook post on Sunday: "EVERYONE I need you all to share as much as possible about my sister. ... More noise that gets made puts more pressure on the island to find her."

Police officers and foreigners, part of the search team, dive during a hunt for missing British backpacker Amelia Bambridge at Koh Rong, in Preah Sihanouk province, Cambodia. Picture: Sihanoukville province Authority Police via AP

Bunthol said Bambridge's brother as well as her father and mother had arrived in Cambodia on Sunday.