The mother of missing British trekker Gareth Huntley has arrived in Malaysia to "be here when they find him".
Mr Huntley, 34, of Cricklewood, north London, has not been seen since going trekking on Tioman Island on Tuesday.
Prime Minister David Cameron has spoken to his Malaysian counterpart about the search.
Over 100 people have been looking for Mr Huntley, but the Malaysian government was criticised for its initial lack of action.
Mr Huntley's mother, Janet Southwell, said she believed her son had been injured but was still alive.
She arrived in Malaysia on Monday and said she wanted to join the search for her son.
She told the BBC: "I want to see the area for myself, absolutely, get a feel for where he's lost, where he's missing.
"I just want to be here when they find him, I think he will want to see a familiar face when they find him."
The British Government said it had been assured "all available search assets" were involved in the hunt, which includes police officers and rescue teams.
The Malaysian government has insisted all available resources are being used in the search after Mrs Southwell criticised their initial efforts.
A spokesman for Number 10 said David Cameron had spoken to prime minister Najib Razak of Malaysia and thanked him for the efforts made by authorities over the past few days to find Mr Huntley.
Mr Cameron was considering what more the UK could do to help, added the spokesman.
A local Foreign Office diplomat has arrived to co-ordinate the rescue effort from the ground.
Emotional 'roller coaster'
Mr Huntley set off to visit a waterfall on the island, off the south-eastern coast of the country's mainland, on Tuesday.
He told friends he would be back at the charity project where he was volunteering by 14:00, but failed to return.
Ms Southwell, from Bradford, has been critical of efforts made so far to find her son, but told BBC 5 live she hoped the search would resume at daylight.
"My grateful thanks to David Cameron and the British government. I'm sure their involvement has really helped things," she said.
"It's a roller coaster of emotions. You know it's despair one minute and hope the next and back again.
"Gareth's quite a resilient character, he's a determined young man and I'm feeling really positive that with all this effort we are going to find him.
"He may be injured but we are going to find him and we will bring him out alive."
Mr Huntley's girlfriend Kit Natariga, who has also flown out to the island, wrote on Facebook that it was "absolutely crucial to act fast and continue to push the authorities and rescue teams".
But she admitted time to find him was "running out".
Last week, Mr Huntley's family raised concerns about the rescue effort.
His brother Mark had said police did not initially join the search and for days it was only his friends and local people who were looking for him.
Ms Southwell also appealed directly to Prime Minister David Cameron to put pressure on the Malaysian government to step up the search.
Defence Secretary Philip Hammond, who is in Malaysia, raised the matter with his counterpart on Sunday.
He "was assured that the Malaysian authorities would do all they could to locate him and had already deployed significant assets", the Foreign Office said in a statement.
On his official Twitter feed, the Malaysian Defence Minister Hishamuddin Hussein also said he was helping to find missing Mr Huntley.
Steve Riches, who has attempted the trek to the waterfall, said: "It says two hours but once you get beyond the rubber tree hinterland, you enter the jungle proper and the path is not clearly marked.
"We crossed a very boulder-strewn river bed and then realised we were lost and with some difficulty, we retraced our steps until we hit the track.
"It is slippery and you can take a fall - I did - and we just played safe and came back to the start. Others say they did it ok."
He added: "Once you've headed off one of the few tracks, and are away from the rubber plantation strip, you really are in difficult virgin jungle with rattan palm that can rip your skin to pieces.
"It can be so dense that even a companion 15m (50ft) away cannot be heard.
"I saw a python, and there were vipers, and you get covered with bloodsucking leeches, but frankly the greatest dangers are not from nature but from falling or just being completely lost without water."
BBC News correspondent Jennifer Pak, who is in Kuala Lumpur, said the area was a dense jungle.
"It's extremely difficult because they've been experiencing rainfall every night, [and there is] more to come in the coming days," she said.
"Also the search has been limited to daylight hours so as soon as it gets dark at 19:00 local time, it has to stop.
"Even if the pathway he was using was well trodden, it's extremely wet, slippery, so it would have been very difficult."