Two United States Government investigators were last night travelling to Japan in an effort to find the cause of the worst air crash involving a single aircraft.

Japan Air Lines said that 524 passengers and crew, including 21 non-Japanese, were feared killed when one of its Boeing 747 jets crashed into mountainous terrain north-west of Tokyo.

Flight 123, flying a domestic route from Tokyo to Osaka, apparently veered off course shortly after taking off for its 60-minute journey. It was hundreds of miles away from its planned route when it came down.

Early today, 70 paratroopers descended on ropes from helicopters to start hunting through the wreckage. But Defence ministry officials said there was no sign of survivors. About another 1,000 emergency workers were trying to reach the scene on foot.

Rescuers had great difficulty reaching the remote Japan Alps, 70 miles north-west of the capital, and heavy rain added to their problems.

The region is difficult to reach even on foot, intersected with gorges and densely-packed fir and spruce trees. Few roads run through it.

'One helicopter found what looks like the tail of the aircraft,' a defence ministry spokesman said. Debris was scattered over an area of at least three miles.

A housewife in Nagano Prefecture, near where the plane is believed to have crashed, told the Japan Broadcasting Corporation: 'An aircraft flew over my house headed south. The aircraft was lower on the left side and appeared to be on the verge of falling.

The decision to send officials of the US National Transportation Safety Board followed reports that the pilot had had problems with a rear door. He radioed that the seal had burst: a spokesman for the board in Washington said that this was the first time such a problem had ever been reported with the 747.

In 1974 the loss of a badly-fastened rear cargo door caused the crash of a Turkish Airlines DC-10 near Paris, with the death of 346 people.

The JAL pilot, Captain Masami Takahama, aged 49, reported difficulties soon after departure. He gave his position as 55 miles south-west of Tokyo, which would put him on course for Osaka.

He told ground controllers that a seal had given way on one of the doors and that his plane was dropping below the 24,000 feet assigned for his flight. A little later he radioed that he could not control the plane and that he had no idea of his position.

The aircraft, a special short-range version of the 747, was fully loaded, mainly with Japanese travelling to celebrate the religious festival of Obon, when it is customary for urban Japanese to go to their rural homes to pay respect to family ancestors. Twelve infants were reported to be on the passenger list.

The pilot was told that he could make an emergency landing at a nearby US air force base, but his interchanges with the air traffic controllers appear to have become confused at this point.

No reply was received to the emergency instruction but later, when asked if he wanted to return to Tokyo airport, Captain Takahama was reported to have said, 'Yes' in an excited voice.

The pilot of a nearby plane said the jumbo pilot sounded short of breath and that his voice was muffled as if he was wearing an oxygen mask. Clearly lost and apparently not in full control of his plane, the pilot hit the north side of Mount Ogura about 2,000 feet below the summit. Japanese meteorologists said the area was affected by thunderstorms at the time of the crash.

Among those who were said to have caught the flight was one of Japan 's most popular singers, Kyu Sajamoto. He became known to Western audiences in the 1960s with his hit record Sukiyaki.

Captain Takahama was one of JAL's most experienced pilots. He joined the airline in 1966 and has logged some 12,000 flying hours. Initial reports of his radio transmissions left it unclear whether he was telling the ground controllers that he was 'unable to control' the plane or that he was experiencing 'unreliable control.'

The involvement of such an experienced pilot and the 747's reputation as one of the world's most successful and reliable civil aircraft lies behind the immediate public involvement of the American federal authorities. It was a swift demonstration of the general concern aroused by the accident in the aviation world. Nearly 500 similar aircraft are in worldwide service.

Because Boeing is an American company the US Government has wide powers to order special safety precautions. In 1979, when a DC-10 crashed on take-off at Chicago airport after losing one of its engines, the Federal Aviation Authority ordered a worldwide grounding of the plane until it was clear that no others in service faced the same risk.

A spokesman for the manufacturers in Seattle said that investigations of previous accidents involving the 747 'have shown that in none of them was the aircraft at fault.'

Aviation experts in London last night speculated that the reported trouble with the door and the loss of the aircraft might be coincidental. There was no evidence of explosive decompression as the pilot communicated with the ground.

Witnesses who saw the plane in its final moments said that it appeared to be making 'a long turn,' a manoeuvre which could have been an attempt by the crew to get back on course over mountainous country after receiving details of their real position from air traffic controllers.

Efforts to recover more wreckage from the Air India jumbo jet that crashed off the coast of southwest Ireland will begin in later this week.

The Canadian coastguard vessel, John Cabot, carrying special equipment, has been delayed in Cork harbour by bad weather. A spokesman for the recovery team said yesterday they hoped to head within days for the crash site, more than 100 miles off County Kerry.

They aim to gather debris that might reveal the cause of the crash - and could take on a fresh significance following yesterday's Japanese 747 disaster.

The voice and digital flight recorder units from the Indian airliner's 'Black box' were located last month.