An alligator at least 10ft long lunged at a teenager swimming in a river and bit off his right arm below the elbow, Florida state wildlife officials said.

Kaleb Langdale, 17, survived the encounter in the Caloosahatchee River.

Wildlife officers who caught and killed the alligator retrieved the arm, but doctors were unable to reattach it.

"We found the alligator that was responsible," Florida Fish and Wildlife Conservation Commission spokesman Jorge Pino said. "We were able to kill the alligator and dissect the alligator, remove the arm and transport the arm to the hospital to see if the doctors could reattach the limb."

The teenager was rushed to a Fort Myers hospital.

His friends told Fort Myers television station WBBH that while the arm could not be reattached, Mr Langdale was in good spirits.

The alligator went straight for the teenager as he was swimming, Matt Baker said.

"It came at him and he put his arm in the way instead of letting it get to his body. It took his arm and him under," Mr Baker said.

Another friend said Mr Langdale popped out of the water shortly after being bitten.

"He was waving saying, 'Call the paramedics! My arm is gone!'" Gary Beck said.

Alligators are more active at this time of year because it is their mating season, which makes them more aggressive and inquisitive as they are looking for food and for mates.

Wildlife officers warn that alligators can call just about any body of water in Florida home.

It is rare for wild alligators to bite humans though, Mr Pino said.

Since 1948, 224 people have suffered major alligator bites, including 22 fatal bites, according to June 2011 conservation commission data.

Wildlife officials were investigating what caused the alligator to bite Mr Langdale.

Last month, an airboat captain was giving a tour in south-west Florida when a 9ft alligator bit off his left hand.

The Indiana family on the boat said the captain had hung a fish over the side of the boat and had his hand at the water's surface when the alligator bit him.