A deep-sea robot has located pieces of the missing EgyptAir plane at the bottom of the Mediterranean as investigators race to find the black boxes that could reveal the cause of the crash.

While the wreckage discovered may offer clues about why the Airbus A320 went down with 66 people on board nearly a month ago, its manufacturer said Thursday the flight recorders held the key to unlocking the mystery.

"The first photos of the wreckage do not allow to establish any scenario of the accident," an Airbus statement said.

"Only the black boxes could contribute to a full understanding of the chain of events which led to this tragic accident."

Investigators have said it is too soon to determine what caused flight MS804 from Paris to Cairo to crash on May 19, although a terror attack has not been ruled out.

The search vessel John Lethbridge, equipped with an underwater robot, arrived in Egypt last week to begin searching an area around 290 kilometres (180 miles) north of the Egyptian coast.

The robot discovered pieces of the fuselage at "several sites", the Egyptian board of inquiry said late Wednesday.

A source close to the investigation told AFP that the robot, operated by Mauritius-based Deep Ocean Search, had found "small fragments" of the plane.

Some wreckage had already been pulled out of the Mediterranean by search teams last month, along with belongings of passengers.

The "pings" emitted by the black boxes were detected by French survey ship Laplace on June 1 but the flight recorders' exact location has not yet been established.