Malaysia's opposition on Monday denounced what it called a government bid to increase the prison term of opposition leader Anwar Ibrahim in a controversial sodomy case.

Anwar, 66, was sentenced to five years' jail last month after a court overturned a 2012 acquittal on sodomy charges that he says are a false smear by Malaysia's authoritarian regime aimed at disqualifying him from politics.

Australia decided Monday to deploy a mini-submarine to scour the unmapped Indian Ocean seabed for Malaysian jet MH370 at a daunting depth of 4,500 meters (15,000 feet), ending the search for black-box signals.

Angus Houston, who heads the Joint Agency Coordination Center, also revealed that an oil slick had been sighted in the area of the search led by the Australian vessel Ocean Shield far off Perth.

Malaysia on Sunday rejected claims that phone calls were made from missing flight MH370 before it vanished, but refused to rule out any possibility in a so far fruitless investigation over the cause of the jet's disappearance.

The New Straits Times, quoting an anonymous source, had reported Saturday that co-pilot Fariq Abdul Hamid made a call which ended abruptly, possibly "because the aircraft was fast moving away from the (telecommunications) tower".

Three Muslim rebels and two soldiers were killed Friday in a day-long gunbattle between troops and al-Qaida-linked Abu Sayyaf extremists blamed for beheadings and kidnappings in the southern Philippines, the military said.

The fighting began as part of an operation to capture Abu Sayyaf commander Puruji Indama, military spokesman Lieutenant Colonel Ramon Zagala said in a brief statement, and sporadic gunfire continued into the night.

Australian Prime Minister Tony Abbott said Friday he is "very confident" that signals detected in the search for Flight MH370 are from the aircraft's black box, whose batteries are waning fast more than a month after the plane vanished.

"We have very much narrowed down the search area and we are very confident the signals are from the black box," Abbott said, while warning that the transmissions were "starting to fade."

Kidnappers based in the southern Philippines have demanded $11.25 million in ransom for a Chinese tourist abducted from a Malaysian dive resort, a senior minister said Thursday.

Malaysian Home Minister Zahid Hamidi told Agence France Presse the family of Gao Huayun, who was kidnapped by gunmen on April 2 along with a Filipino resort worker, are negotiating with her abductors through an intermediary.

Two fresh signals have been picked up in the search for missing Malaysian flight MH370, raising hopes Wednesday that wreckage will be found within days even as black box batteries start to expire.

Australian ship Ocean Shield detected the signals Tuesday to match a pair of transmissions picked up over the weekend that have been analyzed as consistent with signals from the plane's flight data recorder, the head of the search said.

An Australian navy ship has detected new underwater signals consistent with aircraft "black boxes", the chief of the MH370 search said Monday, calling it the "most promising lead" yet in the month-old hunt for the missing plane.

Retired Australian defense force chief Angus Houston said the acoustics emanating from deep down in the Indian Ocean showed that the multinational search by ships and planes seemed to be "very close to where we need to be".

Australia on Sunday sent planes and ships to investigate signals detected by a Chinese ship in the hunt for a missing Malaysian jet, saying they matched black box beacons and were an "important and encouraging lead".

Angus Houston, the Australian head of the mission, said a second "ping" was also being scrutinized 300 nautical miles away in the Indian Ocean, as the one-month lifespan of batteries powering the beacons loomed.

A Chinese ship involved in the Indian Ocean search for flight MH370 detected a "pulse signal" at a frequency used by aircraft black boxes Saturday, but Australia warned there was no evidence yet that it was linked to the missing plane.

China's Xinhua news agency said the signal picked up by a black box detector on the vessel Haixun 01 had a frequency of 37.5kHz -- identical to the emergency beacon signal emitted by flight recorders.