China's wild finless porpoises are heading toward extinction, a conservation group said Thursday, with the dolphin-like animals now rarer than the giant panda.

With a stubby nose and grey body, the porpoises inhabit the Yangtze River and are famed for their cuteness in China, where they are known as "river pigs".

But their numbers in the Yangtze, which is the country's longest river, have more than halved in six years, according to an extensive survey.

Scientists spent over a month last year scanning more than 3,400 kilometres (2,125 miles) of the river in a hunt for the porpoises, but only saw 380, the conservation group WWF said in a statement.

Based on that observation, combined with sightings of the porpoises in lakes connected to the river, the total number alive in the wild was likely to be a little more than 1,000, the WWF said.

There are around 1,600 giant pandas living in the wild, according to the WWF, which has said the porpoise could become extinct in 15 years if no action is taken.

The species is "moving fast toward its extinction," the WWF quoted Wang Ding, head of the research expedition, as saying.

The finless porpoise, which unlike the dolphin has a small dorsal ridge rather than a fin, has been hurt by human intrusion and environmental degradation.

"Food shortage and human disturbance such as increased shipping traffic are the major threats," the WWF said, adding that researchers also discovered "traps that could affect finless porpoises".

Waterways in China have become heavily contaminated with toxic waste from factories and farms -- pollution blamed on more than three decades of rapid economic growth and lax enforcement of environmental protection laws.

Environmental activists also say the huge Three Gorges Dam and other hydropower projects on the Yangtze have upset the delicate ecological balance and harmed aquatic life in the river.

The survey failed to find any trace of the Baiji Dolphin, a close relative of the finless porpoise that was declared "functionally extinct," after a survey in 2006.