Malaysian researchers have discovered a new amphibian species during an expedition to the rainforests of Mount Singai in the country’s state of Sarawak.

The 4 to 5 centimeters (1.6-2.0 inches) long brown frog was discovered on Borneo Island, known for its rich biodiversity, said Indraneil Das of the Universiti Malaysia Sarawak. 

"We heard a call we hadn't heard before. It called from under the leaf litter. That's probably why no one saw it before," Das explained, adding that the call was very distinctive, high-pitched, loud and repeated. 

The team found the frog in September 2010, but the result has been recently announced as discovering new species requires a lengthy scientific process. 

Das said that other unknown frog species, earlier found by the team, are still under the investigation process. He added that these species were also discovered on Borneo Island which is shared with Indonesia and Brunei. 

Das has also previously discovered Asia's tiniest frog in size of a pea, in Kubah National Park in Malaysia.