Friends of a British teenager who disappeared in Tenerife have urged British and Spanish police to send reinforcements to aid the search as they have resorted to hunting for evidence and interviewing potential witnesses themselves.

Police on the island have been looking for Jay Slater, a 19-year-old apprentice bricklayer from Lancashire, since Monday morning with the help of drones and a helicopter.

But the area where he disappeared, in Rural de Teno nature reserve, is so vast and rugged that friends have said specialist search teams from the UK and the Spanish mainland need to be drafted in to help.

Slater’s friend Lucy, with whom he had been on holiday, was among those urging UK police to fly out to help with the search.

She reported Slater missing after she received a call from him at about 8.15am on the morning he disappeared, in which he told her that he did not know where he was, that he was thirsty and had just 1% phone battery.

“I just think they need to get as much help as they can out here,” she told the Guardian. “There’s no helicopters here 1718957323. He’s been missing for that long now, what are they actually doing?”

Lucy said she welcomed the support from local police “who know the area”, but said the search operation needed to be much more intensive. “There’s nobody here,” she said.

On the first day she reported Jay missing, Lucy said she had gone back to his last known location. “I was here literally until it was dark, and I was told I needed to come away,” she said. “That full day not one person was here.”

In efforts to help find him, friends have identified the house he went back to with people he had met at a music festival the night before he disappeared.

They said they had traced the house from social media videos, and that by speaking to people in the village, they learned that the men whom Slater had befriended had already flown back to the UK.

Slater’s friends do not believe the men were spoken to by police before they left the island to see whether they may hold vital information about his last known movements.

They have also combed the expansive undergrowth, finding bags, clothing, and other detritus probably left behind by tourists, among the caves, rocks, cacti and banana plants that make up the landscape.

They have collected and kept items such as cigarette butts, in case they may later prove useful to the investigation.

On Thursday afternoon there were some police and search and rescue workers in the area where Slater was last known to have been, but there was no sign of any large-scale search operation.

“Even if they had 30,000 people here it’s like a needle in a haystack,” another family friend said. She said they had spent 12 hours on Wednesday trawling all of the paths in the area surrounding Slater’s last known location.

“It needs to be taken seriously,” Lucy added, saying she believed UK police may have specialist resources that could aid local forces.

She also said friends and family needed more support, as they were struggling to get information from, and communicate with, local police.

Speaking to ITV news on Thursday morning, Slater’s mother, Debbie Duncan, described her “living nightmare” as she waited for news about her “beautiful boy”.

She said: “I wouldn’t wish this on anybody. I just want my baby back. Please just anybody who can help – look for him. It’s a massive area up there.

“He’s out there somewhere or somebody knows where he is …

“I wish I hadn’t encouraged him to go to this. I should have said: ‘Don’t go to Tenerife.’

“I just think he was probably still in high spirits, buzzing – he’s not known where he is. He’s not known the extent of the long journey that he went on to get up there.”

One member of the emergency services on site in the national park said search teams had not given up hope, and they had people stationed on all main paths in the area. He said they remained optimistic that someone could survive in current conditions.

A Lancashire police spokesperson said: “Our role is in supporting the family. We have been to see them, and we will continue to offer them support. Obviously our thoughts are with them at this time. We are also in touch with the consulate in Tenerife for any updates.”