His friend Frank Gardner, the BBC's security correspondent who travelled to Papua New Guinea with him twice last year, said Allen had tracked "huge distances" to reach the remote airstrip.
Gardner said: "Benedict Allen is not out of danger yet. He is currently marooned in a remote part of Papua New Guinea that is only reachable by air after all the road bridges were cut due to tribal fighting.
"Urgent efforts are now under way to try to airlift him out as soon as possible in case fighting erupts around him."
The BBC correspondent said he was "quite annoyed with him as a friend" for leaving without a plan.
"I'm sure he's come back with an incredible story to tell which will be fascinating and he'll regale audiences at the National Geographic Society and elsewhere but we could have done without this worry on his behalf," he said.
Allen's agent confirmed on Thursday morning that he had been spotted, to the relief of his wife and family.
In a statement Jo Sarsby said: "At 5pm local time [PNG] Mr Keith Copley, the Coordinating Director for New Tribe Mission in Papua New Guinea, confirmed in writing that Benedict Allen was safe, well and healthy and is presently located at a remote airstrip 20 miles northwest of Porgera, Enga Province.
"Confirmation on exact location coordinates are now being confirmed in order to arrange evacuation as soon as possible."
Gardner told BBC Radio 4's Today programme: "He has been reported alive and well.
"He's been sighted, we've had a second sighting of him now, tribal chiefs in the area that he went missing say that he has been sighted near an airstrip, that he's okay, that his health is fine, that he's requesting rescue.
"So efforts are now under way to try and bring him out.
"It's always possible that they could have made a mistake, but I'm not aware of any other 57-year-old lanky British explorers that have gone missing in that area so let's hope that it isn't a false glimmer of hope but I think he's going to be okay."
Earlier, his wife Lenka Allen, said that their children - 10-year-old Natalya, Freddie, 7, and 2-year-old Beatrice - keep asking "when's Daddy coming home?"
Lenka Allen told the Daily Mail: "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.
"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."
The author and TV presenter, who has made six TV series for the BBC, had no mobile phone or GPS device with him and was expected to begin his journey home at the weekend - but failed to make a flight to Hong Kong.
In a blog post on his website, Allen wrote in September: "The Yaifo are one of the last people on the entire planet who are out of contact with our interconnected world.
"In October I'm hiring a helicopter to drop me off at the abandoned mission station, Bisorio - a forlorn place.
"Last time the Yaifo greeted me with a terrifying show of strength, an energetic dance featuring their bows and arrows.
"On this occasion who knows if the Yaifo will do the same, or run off, or be wearing jeans and T-shirts traded eons ago from the old mission station.
"Nor do I have an obvious means of returning to the outside world, which is somewhat worrying, especially at my advanced age."
He added: "Either I must paddle down river for a week or so - or enlist the help of the Yaifo, as I did last time.
"So, if this website or my Twitter account falls more than usually silent - I'm due back mid-Nov - it's because I am still out there somewhere.
"So, don't bother to call or text. Just like the good old days, I won't be taking a sat phone, GPS or companion. Or anything else much. Because this is how I do my journeys of exploration. I grow older but no wiser, it seems."
A Foreign and Commonwealth Office spokesman said: "Our staff are assisting the family of a British man who has been reported missing in Papua New Guinea, and are contacting the local authorities."
