UPDATE 1-Breakthrough in Air France crash black box search

* Defence ministry says “probably” found black box signals

* Search area still size of Paris, no guarantees of recovery

* Cost of search totals 28 mln euros ($37.6 million) so far

(Updates throughout with defence ministry news conference)

By John Irish

PARIS, May 6 (Reuters) - France has found what could be the first concrete clues to the location of black boxes missing from last year’s Atlantic jet disaster, but warned on Thursday there was no guarantee the breakthrough would lead to their recovery.

The defence ministry said the boost had come through detailed follow-up analysis of sonar readings taken in the first few weeks after an Air France AIRF.PA jet crashed into the Atlantic killing 228 people on June 1 last year.

Finding the black boxes is seen as essential to help crash experts and relatives understand exactly what caused flight 447 to plunge into a remote part of the Atlantic during an equatorial storm on its journey from Rio de Janeiro to Paris.

“It is probably the signal (of the boxes),” said General Christian Baptiste, deputy spokesman at the Defence Ministry.

The discovery of a possible 'ping' from at least one of the recorders on board the Airbus EAD.PA A330 has allowed experts to narrow the search to a few square kilometres from several thousand ahead of the anniversary of the airline's worst crash.

“Does this mean we have found the black boxes? We are still far from certain,” Baptiste said. “The search zone still equates to an area the size of Paris and we have to find an object the size of a shoebox in seabed terrain which looks like the Andes,” he told a news conference.

The recording which could contain the signal emitted from the recorder devices, buried until now behind background noise, was made on July 1, exactly one month after the crash.

Black box flight recorders are designed to emit homing signals for around 30 days.

“It is possible this will help us find important pieces of wreckage, and if we have a lot of luck a black box could be found in one of these pieces of wreckage,” Baptiste said.

Two sophisticated salvage vessels, using miniature submarines, have been scouring a 3,000-square-km area to try to locate the flight recorders of the Airbus EAD.PA A330 plane.

Baptiste said the search was now about 400 kilometres northwest of Brazilian islands of Sao Pedro and Sao Paulo.

Air France said the breakthrough was “excellent news”.

Recovery will depend on the depth where wreckage could lie, anywhere between 1 km (0.6 miles) and 4 km (2.5 miles) down.

A spokeswoman at France’s air accident investigation authority BEA said Air France and Airbus had spent 13 million euros on the inquiry and French state bodies 15 million euros.

Speculation about the cause of the crash has focused on possible icing of the aircraft’s speed sensors, which appeared to give inconsistent readings seconds before the plane vanished. ($1=.7453 Euro) (Additional reporting by Thierry Leveque and Elisabeth Pineau, writing by Sudip Kar-Gupta, Tim Hepher and John Irish; editing by Philippa Fletcher)