Sydney - Four civil jets joined ten military planes Friday in the search for a sign of Malaysia Airlines flight MH370, which went missing nearly four weeks ago, Australian authorities helping coordinate the search said.
The multinational operation in the Indian Ocean is now in its 18th day, with the 217,000-square-kilometre search zone shifting slightly north of previous unsuccessful searches.
Australian volunteers aboard three of the civil aircraft will work as air observers, while the fourth plane will act as a communications relay, the Joint Agency Coordination Centre (JACC) said.
Nine ships and one submarine are also involved, including the Australian navy ship Ocean Shield carrying a special underwater signal detector due to arrive in the area later Friday.
JACC said British navy ship HMS Echo picked up an alert on Thursday but it did not come from the plane's black box, the Australian Broadcasting Corporation reported.
Noises from whales or ships could trigger an alert on the ship's sensitive sonic receptors, JACC said.
The search is intensifying as time to find the black box flight recorder is running out. The battery on the device is due to expire anytime after Monday, but the recorder could continue to emit signals for another week.
The Australian Transport Safety Bureau said it is continuing to work on narrowing down the area where the plane could have hit the water using data supplied by an air crash investigation team with analysts from Malaysia, the United States, Britain, China and Australia.