The American space agency NASA's New Horizons probe has captured faint images of the smallest of Pluto's five known moons, Kerberos and Styx.

The animation released today by the agency contains four images of Pluto's moons taken between April 25 and May 1.

Mission science team member John Spencer, of the Southwest Research Institute in Boulder, Colorado said that New Horizons is now on the threshold of discovery.

"If the spacecraft observes any additional moons as we get closer to Pluto, they will be worlds that no one has seen before", he said.

Goring to closer to Pluto in mid-May, new Horizons will begin its first search for new moons or rings that might threaten the spacecraft on its passage through the Pluto system.

The images sent by the craft are allowing the scientist to refine the techniques that they could use to analyze the data.

Kerberos and Styx were discovered in 2011 and 2012, respectively, by New Horizons team members using the Hubble Space Telescope, said agency.

As per experts, Styx completes a single circle around Pluto in 20 days between the orbits of Charon and Nix. Styx is almost 4 to 13 miles (7 to 21 kilometers) in diameter.

Kerberos on the other hand take 32-day perod to orbit Pluto. It is orbiting between Nix and Hydra and is just just 6 to 20 miles (10 to 30km) in diameter. Each is 20 to 30 times fainter than Nix and Hydra.

The images detecting Kerberos and Styx were taken with New Horizons' most sensitive camera, the Long Range Reconnaissance Imager (LORRI).

Each observation consists of five 10-second exposures that have been added together to make the image in the left panel, said agency.

Kerberos is visible in all of the images, though is partially obscured in the second image, it added.