Elite Indonesian military divers battled strong currents on Thursday in an effort to reach the submerged tail of crashed AirAsia Flight 8501 in the hopes of finding crucial black box data recorders.

The plane crashed on December 28 during stormy weather as it flew from the Indonesian city of Surabaya to Singapore, claiming the lives of all 162 people on board.

Stormy weather has plagued multinational efforts to find the wreckage of the plane in the Java Sea, as well as all of the bodies and the black boxes that should contain the pilots' last words.

The biggest breakthrough came on Wednesday with the discovery of the tail, which is where the black boxes are kept, buried into the seabed 30 metres (100 feet) underwater.

However strong currents stymied efforts on Thursday morning by divers from the Indonesian Marines' elite diving unit to penetrate into the tail, search and rescue agency chief Bambang Soelistyo told reporters in Jakarta.

"Divers have reached the tail part but... the visibility was below one metre so they only managed to retrieve various debris," Soelistyo said.

"Now we are waiting for the speed of the current to ease. If it gets calmer later, they will go back to do another dive to determine whether the black boxes remained in the tail or were detached."

Soelistyo said retrieval experts were also ready with a crane to lift the tail out of the water.

However he said a decision on extracting the tail would not be made until a more through inspection of it was carried out, with the continuing storm weather another factor.

Soelistyo said the other top priority was the search for bodies, with just 40 found so far floating at sea.

Many of the others are believed to be inside the wreckage of the plane's main cabin, which has not been found.

All but seven of those on board were Indonesian.

The search -- involving US, Russian, Chinese and other foreign military assets -- is being conducted from Pangkalan Bun, a town on the island of Borneo which has the closest airstrip to the crash site.

The Indonesian meteorological agency said weather was the "triggering factor" of the crash, with ice likely damaging the engines of the Airbus A320-200.

But a much clearer explanation is not possible without the black boxes.


Poor safety record



Indonesian authorities also said the plane was flying on an unauthorised schedule when it crashed, and AirAsia has since been suspended from flying the Surabaya-Singapore route.

Indonesia's transport ministry said on Wednesday that it had fired one transport official and disciplined several others in a crackdown following the crash.

It was expected to announce this week the results of a deeper investigation into how the flight was able to depart without permission.

Indonesia's air travel industry is booming, with domestic passengers growing nearly five-fold over the past decade and airlines scoring billion-dollar deals with foreign plane makers.

But it has a dismal air safety record.

In 2007, an Adam Air plane plunged into the sea off Sulawesi island on New Year's Day, killing all 102 people on board. That airline was later banned from flying.

A few months later, a jet with flag carrier Garuda Indonesia burst into flames on landing in the province of Central Java, killing 21 people.

Authorities have sought to tighten regulations on the aviation sector since the darkest days of 2007, but have conceded the fact AirAsia was flying on an unscheduled day showed more needed to be done.

AirAsia Indonesia has declined to comment on allegations it violated its permits. Singapore authorities say the Sunday flight schedule had been cleared at their end.

The airline is a joint venture involving Malaysia-based budget carrier AirAsia, which previously had a solid safety record.