A US farmer says he had "no other choice" but to cut off his own mangled leg with a pocket knife after it became trapped in farm machinery.

Key points: Kurt Kaser said he forgot he had removed a safety screen covering the auger's hopper, and got his foot trapped inside

Kurt Kaser said he forgot he had removed a safety screen covering the auger's hopper, and got his foot trapped inside The auger — which looks like a giant drill bit — kept tearing away skin, tissue and muscle and he could see bone protruding from his lower leg

The auger — which looks like a giant drill bit — kept tearing away skin, tissue and muscle and he could see bone protruding from his lower leg After hacking at his leg, Mr Kaser crawled about 45 metres to a phone before being flown to hospital

Nebraskan man Kurt Kaser, 63, was unloading corn last month when he accidentally stepped on the opening of his grain auger and his leg was sucked inside.

He said he had removed part of the safety screen covering the hopper to make it fit under a bin after the ground on his farm had frozen in the winter.

"I'm kind of disappointed in myself that I didn't think of fixing that thing, or whatever. But that's why they call it accidents I guess," he told America's ABC News.

On April 26, he was filling that auger with grain, forgetting he had removed the screen.

Kurt Kaser said he forgot he had removed the safety screen on the hopper of his grain auger. ( KETV )

Mr Kaser said he could not free his leg, he was alone and he didn't have his mobile phone.

"I stepped right in the damn thing. It grabbed a hold of me. I can remember seeing it start and I go, 'This ain't good,'" he said.

"And then when my foot was in there banging around, I was trying to hold my leg, pulling it out, and I said, 'This is not good.'

"I don't know if I had [my mobile phone] with me, if it fell in the auger and went in the bin. I don't know. But nobody has been able to find it."

The auger — which looks like a giant drill bit and ferries grain from the ground to a bin — kept pulling on his leg, tearing away skin, tissue and muscle until he could see bone protruding through the lower portion of his leg.

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So he took out his pocket knife and sawed off his leg below the knee.

"I felt it try to grab me again. I about gave up and said 'whatever happens happens. If it sucks me in all the way, it's over with,'" he said.

"But then, all at once, I thought of my pocket knife."

Mr Kaser said he nearly dropped the knife in the auger, but once he had a good grip, he started sawing off his leg.

He said he doesn't remember experiencing much pain.

"Finally it let go and I got out," he said.

"Adrenaline kicked in so much that I don't know if it hurt or not."

He then crawled 45 metres to a phone and was flown to a hospital. He never lost consciousness.

Mr Kaser will get a prosthetic limb once his leg has fully healed.

ABC/AP