US astronomers have found a fifth moon orbiting far-away Pluto, NASA said on Wednesday.

The irregular-shaped moon, nicknamed S/2012 for now, is about six to 15 miles (10 to 24 kilometers) across, the US space agency said.

Last year, astronomers reported finding the fourth moon around the icy dwarf planet some three billion miles away.

"The moons form a series of neatly nested orbits, a bit like Russian dolls," said Mark Showalter of the SETI Institute in Mountain View, California.

Pluto's largest moon, Charon, is 648 miles across. The other two, Nix and Hydra, are between 20 and 70 miles in diameter, NASA said.

Hubble -- a potent space telescope that has transformed the field of astronomy since it was first launched in 1990 -- discovered Nix and Hydra in 2005. Astronomers at the US Naval Observatory glimpsed Charon in 1978.

Pluto, once known as the ninth planet from the Sun, was declassified as a full-fledged planet in August 2006 and joined the new category, dwarf planet.

At about 1,430 miles wide, it is about two-thirds the size of the moon and has a mass less than one percent of the Earth's.