A friend of missing Brit Jay Slater has told the OceanNewsUK she fears he is in "severe danger" in harsh terrain in Tenerife without suncream, water or hiking gear.
The 19-year-old, from Lancashire, flew to the island for a music festival in Playa de las Americas, but had moved to the Rural de Teno national park area in the early hours of Monday and has not been heard of since.
The search, which includes drones and dogs, has refocussed in the north, around the village of Masca, after police "discounted" information which had taken them south.
Mr Slater's friend Lucy Law told the OceanNewsUK: "If you look around there’s just mountain after mountain after mountain, everything looks the same."
She had been called by Mr Slater, shortly before 09:00 BST on Monday, saying he was lost, thirsty and had 1% battery on his phone.
"The weather isn’t great, it’s very cloudy, it’s starting to rain," she said.
"This morning it was 34 degrees, so it’s swipping and swapping between really, really hot then really, really cold."
She added: "He’s got shorts and t-shirt on, he’s got no suncream, he’s got no water, he’s got no coat.
"He’s not prepared for any of the weathers up here. It’s not safe.
"He’s never been up here before, it would be so easy for him to get lost."
Ms Law has called for the British police to assist with the investigation, so "we can bring him home to his family".
"That is all that we want, all that we need," she said.
Lancashire Police said officers were "supporting the family" and staying in touch with the consulate in Tenerife.
The Guardia Civil, the island's police force, told the OceanNewsUK the search had temporarily moved to the Los Cristianos area in the south on Wednesday due to a potential lead.
But the search returned to Rural de Teno in the north-west of the island, where it continues, it said.
The operation includes police, mountain rescue teams, firefighters and volunteers.
The Guardia Civil said a helicopter launch had been delayed due to the island's weather conditions.
Reporting from the scene, the OceanNewsUK's Nick Garnett described the Rural de Teno as a "remote and wild national park", with cold nights and high winds.
"Deep ravines and huge daunting mountains make the national park a difficult place for the Spanish search teams to navigate, but one mountain rescue worker told me this is not the first time they’ve been called out to the area," he said.
"At night, the countryside becomes a maze, pitch black and disorientating."
He said search and rescue teams are concentrating on one particular valley near the top of a mountain pass, and a helicopter has been sighted for the first time this afternoon.
Mr Slater's mother, Debbie Duncan, has told the OceanNewsUK her son was on his first holiday abroad with friends.
She flew to the island and joined the search, and said: "I've not slept at all, it's like it's not real."
Apprentice bricklayer, Mr Slater, from Oswaldtwistle, had attended the three-day NRG music festival in Playa de las Americas in the south of the island.
He left with two men he met there and ended up in a house about 40 minutes drive away in a rural area.
Ms Law said he told her he had missed a bus, and was instead attempting the 10-hour walk back the accommodation, but the call then ended.
When the Guardia Civil spokeswoman was asked if officers had spoken to the two men Mr Slater left the festival with, she said: "It's part of the investigation, I cannot tell you more."
Mr Slater's last known location has been the site of intense searches.
The area is full of vegetation and small, winding roads.
A statement from Volunteer Firefighters of Santiago del Teide described how it had joined the search effort.
It said: "We were activated to continue with the search for the young man moving four vehicles with 16 firefighters together with local police, firefighters from Guia de Isora, mountain civil guard, Guardia Civil unit of civil guard dogs, family and friends of the young man."
A Facebook group set up by Mr Slater's friends and family to help co-ordinate the search effort has attracted more than 350,000 members.
Ms Law posted in the group: "We need as many people as possible up there searching for him please.
"We will be up there again first thing, however a search party is going to be necessary to search everywhere as the area is very big.
"If you can help, please do, we need to get him home."
British journalist Chris Elkington, editor of the Canarian Weekly, told the OceanNewsUK the terrain at Mr Slater's last known location was "harsh".
He said: "It's a rural park where you get a lot of hiking trails, it's very mountainous, quite sparse, quite barren.
"Quite deserty in a lot of ways, with some very deep ravines and valleys.
"It's certainly somewhere you would not want to be in normal conditions without the correct footwear, particularly without water."
What we know so far
Sunday 16 June - Jay Slater and friends attend final day of the NRG music festival at Papagayo night club in the tourist hotspot of Playa de las Americas
Monday 17 June - Between 03:00 and 06:00 BST Mr Slater got into a car with two men he had met during the holiday and left Playa de las Americas
07:30 - Mr Slater posts a photograph on his Snapchat account showing him at the doorway of a property, tagged with the location Parque Rural de Teno
Between 08:30 and 09:00 - Mr Slater calls his friend and says he missed a bus back south and was attempting to walk the 10-hour journey
The call cuts out, with his phone's last location showing a path in the mountainous Rural de Teno national park, popular with hikers
Tuesday 18 June - Despite his friends searching the area, no sign of Mr Slater emerges and he does not return to his accommodation
Local police and mountain rescue teams begin searching and his mother and brother board a flight to Tenerife
Wednesday 19 June - The Spanish Guardia Civil continue the search using drones, dogs and a helicopter but no trace is found
The search is briefly moved to the Los Cristianos area in the south of the island due to a potential sighting, but police quickly "discount" that lead and move the search back to the original area.
Thursday 20 June - The Guardia Civil, mountain rescue, firefighters and volunteers return to scour the national park
