A Helios Airways Boeing 737 crashed into a mountainous area north of Athens on Sunday, killing all 121 people on board, after apparently suffering a loss of cabin pressure or oxygen.

"The pilot has turned blue," a passenger said in a mobile phone text message to his cousin, according to Greek television. "Cousin farewell, we're freezing."

The crash was the worst airline disaster in Greek history.

Greek TV station Alpha said the pilot told air traffic controllers the 737 was experiencing air conditioning problems, before communication with the plane -- flying at 35,000 feet en route from Larnaca in Cyprus to Prague via Athens -- was lost.

A Greek police spokesman said 115 passengers and six crew were on board the flight, of whom 59 adults and eight children were heading to Athens and 48 to Prague, including two children.

Rescuers said they had found no survivors, and health officials said they had started receiving the first bodies from the burning wreckage, scattered widely about the mountainous area 40 km (25 miles) north of Athens.

Dense black smoke billowed from several small fires. Only the tail section, bearing the Helios logo, was recognizable among the debris.

"I saw many bodies scattered around, all of them wearing (oxygen) masks," one witness said.

Cypriot Transport Minister Haris Thrasou told reporters in Larnaca: "The state of the bodies is such that it is difficult to recognize at first sight... This is why genetic material will be used (for identification)."

Police said 35 fire trucks, eight fire-fighting planes and three helicopters were at the scene, along with 105 special rescue operations officers.

Akrivos Tsolaki, head of the accident investigation committee, told reporters at the crash site the plane's two black boxes -- voice and data recorders -- had been located.

At Larnaca Airport desperate relatives demanded Helios release the passenger list. Some chanted: "Helios are murderers".

Airport officials in Cyprus said flight HCY522 left Larnaca at 9 a.m. (0700 GMT) and lost contact at 10:30 a.m.

Two Greek F-16 fighter jets were scrambled after the plane lost contact with the tower at Athens Airport.

One of the F-16 pilots reported that he could not see the captain in the cockpit and his co-pilot appeared to be slumped in his seat, a government spokesman said.

The ministry said it suspected the plane's oxygen supply or pressurization system may have malfunctioned.

"We do not have the slightest indication that there was a terrorist act," said Thrasou in Larnaca.

Greek Defence Ministry officials said 90 minutes elapsed between the alert first being raised at 10:30 a.m. and the plane crashing at 12:03 p.m.

A source said the F-16 pilots were being flown to Defence Ministry headquarters for debriefing. "Their testimony is crucial for the continuation of the investigation. They are the ones with the last visuals of the plane."

A Helios spokeswoman in Larnaca said: "We have no information about any problem with the AC (air conditioning) system. This plane received maintenance as usual and left Cyprus without any problems."

Kieran Daly, editor of Air Transport Intelligence, said the cause of the crash was a "puzzle". "There are very good procedures in place for dealing with a lack of oxygen. There are so many warning systems, the crew should have been aware there was a problem," he said.

"The passenger commenting that it was cold suggests there was no air circulating in the cabin at all, or in the cockpit."

A spokesman for the European Aviation Safety Agency, Daniel Holtgen, based in Cologne, Germany, said: "It is highly unlikely that the loss of cabin pressure alone would cause such an incident. There would have to be other contributing factors."

Greek Prime Minister Costas Karamanlis broke off his holiday to return to Athens, and Cyprus declared three days of mourning.

In Larnaca, Cypriot President Tassos Papadopoulos said: "I express my deepest sorrow and grief for the loss of lives of our compatriots and on behalf of the government I express deepest sympathy to all the relatives. We are standing by them and offering our support."

At the airport in Prague, where friends and relatives had been gathering to meet the flight, screens showing arrivals read simply "delayed".

Helios was Cyprus's first private carrier, established in 1999. It flies to Dublin, Sofia, Warsaw, Prague, Strasbourg and several British airports using a fleet of Boeing 737 aircraft.

Libra Holidays Group, one of Britain's leading independent tour operators, bought Helios in November 2004.

Problems involving Helios planes have been reported in the past. In December 2004, three passengers were taken to hospital after a plane lost cabin pressure and made an emergency landing at Larnaca.