BEIJING (AFP) - A Chinese ship searching for Malaysia Airlines Flight MH370 detected a "pulse signal" in the southern Indian Ocean on Saturday, but there was no evidence yet that it was linked to the missing plane, state media said.

The signal picked up by the vessel's black box detector had a frequency of 37.5kHz per second, the official Xinhua news agency said - identical to the beacon signal emitted by flight recorders.

The announcement came nearly a month after the Malaysian jetliner disappeared off radar screens en route from Kuala Lumpur to Beijing with 239 people on board, triggering an unprecedented international search.

Australian and British vessels are currently involved in a round-the-clock underwater search in the southern Indian Ocean, hoping to pick up a signal from the plane's black box recorder, but the battery powering those emissions is nearing the end of its roughly 30-day life span.

The Chinese search ship Haixun 01 picked up the pulse signal at about 25 degrees south latitude and 101 degrees east longitude, Xinhua said in a brief dispatch.

"Suspected pulse signal picked up by Haixun 01 has not been identified yet," the China Maritime Search and Rescue Centre said on a verified microblog.

Australian Defence Minister David Johnston said he had not received a report on the signal and warned that it may not be from the plane.

"This is not the first time we have had something that has turned out to be very disappointing," he told ABC television.

"I'm just going to wait for (JACC chief) Angus (Houston) and the team and my team to come forward with something that's positive because this is a very very difficult task."