A pair of rare Sumatran tiger cubs has been born at the Smithsonian's National Zoo in Washington, D.C., under a programme designed to help the species survive.

"This birth is really good news because we can't afford to have these species go extinct," said National Zoo biologist Craig Saffoe.

"All I can do is smile because the team has realized our goal of producing critically endangered tiger cubs," Saffoe said. The zoo said fewer than 500 Sumatran tigers remain in the wild.

The cubs were born on Monday to Damai, 4, a first-time mother, and 12-year-old Kavi, under one of about 15 species surival programs, coordinated through the Association of Zoos and Aquariums, to breed Sumatran tigers, Saffoe said.

Under breeding programmes, it can take up to five years from tigers being matched and mated until a cub is born, Saffoe said. Dalai was born at the San Diego Zoo in a conservation project four years ago, and Kavi came from the Atlanta Zoo.

The last Sumatran tiger born in the national zoo was in 2006.

While North America is at the forefront of these programmes with 60 Sumatran tigers, U.S. zoos partner with European and Japanese zoos to ensure successful mating.

The cubs mostly likely will not be on exhibit until fall, but they can be viewed online through webcams in the tigers' den.

The zoo webcams: http://nationalzoo.si.edu/Animals/GreatCats/Tiger/tiger-cubs.cfm?cam=lc4