A critically endangered Sumatran tiger has given birth to three cubs at an Indonesian zoo, a veterinarian at the facility.

"She gave birth naturally, without human intervention. The three cubs are all healthy. Two are male, while we haven't been able to get close to the other to identify it," Suci Terawan, a vet at Medan Zoo in northern Sumatra, told AFP.

The 13-year-old Sumatran tiger named Manis, or Sweetie in English, gave birth to the cubs on October 18, just over a year after she successfully bore three male cubs, Terawan said.

"This is our latest contribution in conserving the critically endangered species," he said, adding that the zoo now has six cubs, and two male and one female adult.

Earlier this year a Sumatran tiger at a zoo on the island's Jambi province gave birth to three cubs, but only two survived.

Fewer than 400 Sumatran tigers are left in the wild, conservationists say, with several dying each year as a result of traps, poaching and other human intervention.