British explorer Benedict Allen, who went missing in a remote Papua New Guinea jungle, was found "alive and well" Thursday. The 57-year-old was spotted near an airstrip in Papua New Guinea, Frank Gardner -- who met up with Allen just before he left for his expedition -- wrote on Twitter.

Allen was dropped off by a helicopter in the jungle in the northwestern part of the country known as the Central Range three weeks ago and concerns about his whereabouts sparked after he failed to arrive in Hong Kong where he was due to talk to the Royal Geographical Society on Tuesday.

A search for Allen was immediately launched and according to reports rescue teams were sent to pick him up after he was located.

On Wednesday, Allen's wife Lenka said their three children are "seriously worried" about their father. She told the Daily Mail their children — 10-year-old Natalya, Freddie, seven, and two-year-old Beatrice — regularly ask when their father is coming home.

"The little one, Beatrice, is always saying 'Daddy' and she's trying to telephone him on my mobile, looking at his photo on the screen," she said. "They all sense the tension in the flat and they are worried deep down. The two other ones, they are saying: 'When is daddy coming so we can go shopping, just me and you, and Daddy can babysit', that sort of thing. But of course, now they know he's in danger, they are seriously worried."

She added that "everything possible" is going through her head, fearing he had been bitten by a snake, got lost or contracted an illness — but added: "He does know a lot about the jungle."

In September, Allen said he had made the "first outside contact" with the Yaifo tribe 30 years ago and was returning "to create a brief record of their lives, if possible tracking down some of those... who'd remember me."

"No outsider has made the journey to visit them since the rather perilous journey I made as a young man three decades ago," he wrote on his website. "This would make them the remotest people in Papua New Guinea, and one of the last people on the entire planet who are out of contact with our interconnected world."

In his last tweet on Oct. 11, Allen wrote that no one can get in touch with him as he is headed for the journey. He shared a cryptic message saying "don't try to rescue me, please."

On Wednesday, Allen's sister, Katy Pestille, told Sky News that it was the second time the explorer had gone missing.

"When he was younger he went from the mouth of the Amazon to the mouth of the Orinoco, through all this uncharted terrain. He was the first person to do that. It's a long, long way and he was absent for six months and missing for three, so we've been through all this before," she recalled.

"Not knowing where he was for three months was really, really awful. This was when he was about 22. Our parents were alive then and it was agony at the time," she added. "We are just sitting and waiting and just hoping that he'll just walk out of a bush, which is what happened last time."