Cambodian officials looking for a British woman missing on Koh Rong island are focusing their search on fishing boats and have interviewed six men about her disappearance.

Amelia Bambridge, 21, from Sussex, was last seen at a beach party at about 3am on Thursday. Nearly 150 Cambodian officials and seven volunteers continued searching the island and surrounding areas on Tuesday, the fourth day of large-scale searches.

Officers have been speaking to six Cambodian men. “They are being questioned,” the provincial deputy police chief, Nop Panha, told AFP. “We are not drawing any conclusions yet.”

It is believed the men were being interviewed as potential witnesses rather than as suspects in any crimes. On Monday a Koh Rong official working with police said that there had been no arrests related to the search.

Facebook Twitter Pinterest Amelia Bambridge, 21, who has gone missing in Cambodia. Photograph: Sihanoukville Province Authority Police/AP

The Guardian understands that at least one of the men being questioned worked at a hostel on Koh Rong. On Monday Harry Bambridge, Amelia’s brother, was accompanied by police as he searched the man’s possessions in the hostel.

Harry Bambridge has complained about a lack of support from the British government regarding the search. He said the family met police in their HQ in Sihanoukville on Tuesday morning. He wrote on social media: “Everyone’s [Cambodian officials] said that they’re not going to stop until she’s found, so I feel a little bit more positive.”

On Monday Amelia’s father, Phil Bambridge, said he was losing hope she would be found alive. Sniffer dogs from Phnom Penh were expected to arrive on Koh Rong on Tuesday to help with the search. Extra boats were being dispatched from Sihanoukville for the ocean search.

Officials and volunteers have placed added focus on checking fishing boats that may have been near Police Beach, where Bambridge went missing. Her backpack, containing items including her bank cards and phone, was found on rocks by the beach on Thursday morning.

Ben Warner, a leader of the volunteer search team, said: “We’re running out of areas where we can look on land, so we’re trying to look at ways that she could have got off the island.”

Laura Link, another leader of the volunteer search team, appealed for anyone with photos of boats around Police Beach to get in contact.

A police spokesperson said: “At this point it’s still an all-area search. They’re searching under the rocks in the ocean and on top of the ocean, but now they want to get in the jungle. All departments are here today: police, navy, military.”

Ryan Harris, 18, from Worcester, travelled to Koh Rong with Bambridge on 19 October after befriending her in Phnom Penh. He spent Monday with her family on the island and appealed for anyone with information to come forward.

“We’re appealing for information from anyone who was at the party then [Wednesday night] or in the last few months, anyone who might have seen anything sketchy or suspicious,” Harris said.

He added: “The family are doing well considering the circumstances. They just want to know what happened, same as everyone else.”