A white tiger on Tuesday attacked and killed a youth who apparently jumped into its enclosure at a zoo in the Indian capital.

Witnesses said the tiger grabbed the young man by the neck as horrified onlookers at the National Zoological Park in New Delhi screamed and threw sticks and stones to try to save him.

One witness said he raced to the enclosure after hearing screams to see the victim locked in the tiger's jaws, "writhing badly in pain".

"He kept suffering for the next 10-15 minutes, but nobody helped him," Himanshu, who only gave one name, told the CNN-IBN news channel.

The head of the zoo told AFP that a guard posted at the enclosure raised the alarm, but the victim was dead before anything could be done.

"We do have tranquilising guns but they are at the zoo hospital. By the time we could have organised those, he was killed," said zoo director Amitabh Agnihotri.

The man climbed over the railing and into a deep concrete ditch at about 1pm, a statement from the zoo said, from where the tiger apparently dragged the man into its grassy enclosure.

Witnesses had said the railings around the tiger enclosure were low and speculated that the victim may have fallen in.

But Agnihotri defended the zoo. "There are guards there. If we increase the height of the fencing, how will people be able to look at the animal?" he said.

The zoo said the victim was a Delhi resident and about 20 years old, contradicting earlier reports that he was a teenager.

One official said he also appeared to have been "under the influence of alcohol".

"Security had tried to keep him away from the enclosure, but he somehow managed to jump in and the white tiger who was present at the enclosure leapt on to him," junior official Rohit Kumar told AFP.

"This is the first time such an incident has occurred in this enclosure."

- Tiger taken away -

Photos posted online of the attack show the tiger standing over the victim, who was lying on the grass curled into a ball and trying to protect his head with his hands.

Another witness said the tiger kept "roaming around" the enclosure, holding the victim by the neck.

"The tiger first caught his neck and then left him before coming back again and taking him to the other side (of the enclosure) by his neck," Bitto, who only gave one name, told TV networks.

The victim's body was later taken away after lying in a corner of the enclosure, draped in a white sheet, while the tiger had been taken away and locked up, an AFP reporter said.

The zoo has at least three white tigers, although only one of them is let out into the enclosure daily for visitors to see, a zoo official said.

White tigers are found in southern and eastern Asia, particularly India, and owe their appearance to a recessive gene. They are regarded as an endangered species.

India is home to 1,706 Royal Bengal tigers and fewer than 100 white tigers, according to the last census in 2011. All the white tigers are in captivity.

Rampant poaching and loss of habitat due to human encroachment are cited as the major challenges to tiger conservation efforts.