A plane carrying 71 people on a flight from Bangladesh swerved erratically and flew dangerously low before crashing and erupting in flames as it landed in Kathmandu, Nepal's capital, killing at least 50 people, officials and witnesses say.

Key points: Plane believed to have approached runway from wrong direction

Plane believed to have approached runway from wrong direction Most passengers from Bangladesh and Nepal

Most passengers from Bangladesh and Nepal Several deadly crashed have happened at Kathmandu's airport

A top airport official said the pilot of US-Bangla Airlines flight BS211 did not follow landing instructions from the control tower and approached the airport's one runway from the wrong direction.

"The airplane was not properly aligned with the runway. The tower repeatedly asked if the pilot was OK and the reply was 'yes'," said Raj Kumar Chetri, the airport's general manager.

But a recording of the conversations between the pilot and air traffic controllers — posted by the air traffic monitoring website liveatc.net — indicated confusion over which direction the plane should land on the airport's single runway.

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Just before landing the pilot asks "Are we cleared to land?"

Moments later, the controller comes back on, using a tone rarely heard in such conversations — perhaps even panic — and tells the pilot: "I say again, turn!"

Seconds later, the controller orders firetrucks onto the runway.

The exact number of dead and injured remained unclear amid the chaos of the crash and the rush of badly injured people to nearby hospitals, but Brigadier General Gokul Bhandari, the Nepal army spokesman, said it was clear at least 50 people had died.

Officials at Kathmandu Medical College, the closest hospital to the airport, said they were treating 16 survivors.

Amanda Summers, an American who works in Nepal, watched the crash happen from the terrace of her home office, not far from the airport.

"It was flying so low I thought it was going to run into the mountains," she said.

"All of a sudden there was a blast and then another blast."

Seventeen injured passengers have reportedly been rescued and sent to different hospitals for treatment. ( Reuters: Navesh Chitrakar )

Selim, the US-Bangla manager, said the plane had circled the airport twice as it waited for clearance to land.

Nitin Keyal was about to board a domestic flight when he saw the plane coming in.

"It was flying very low," said Mr Keyal, a medical student.

"Everyone just froze looking at it. You could tell it wasn't a normal landing."

He said it landed just off the runway, broke apart and burst into flames.

"For a few minutes no one could believe what was happening. It was just terrible," he said

Naren Shrestha, a photojournalist based in Nepal who was one of the first on the scene, said there were many bodies on the ground covered in cloth.

"The Nepalese Army was trying to retrieve dead bodies from inside the plane," Mr Shrestha said.

"The plane was 70 per cent burnt. I took a walk around the crash site and saw belongings of the passengers, gifts.

"I saw some passenger seats and Nepali passports on the ground."

Mr Shrestha said he watched as soldiers pulled a conscious survivor from the wreckage an hour after the crash.

Most of the injured were brought to Kathmandu Medical College, the closest hospital to the airport, where relatives wept as they awaited news.

US-Bangla spokesman Kamrul Islam said the plane was carrying 32 passengers from Bangladesh, 33 from Nepal and one each from China and the Maldives.

He said there were also four crewmembers on board.

'A difficult airport for landing'

Sorry, this video has expired Nepal senior news editor Devendra Subedi speaks to the ABC's The World (Photo: Reuters/Navesh Chitrakar)

Kathmandu's airport has been the site of several deadly crashes.

In September 2012, a Sita Air turboprop plane carrying trekkers to Mount Everest hit a bird and crashed shortly after take off, killing all 19 on board.

Devendra Subedi, senior news editor at Nepal Television, said it is a very busy airport given that it's almost in the city itself.

"Weather-wise, Kathmandu hasn't been great for the last two days … afternoon time, clouds had been building up and the pre-monsoon activity has been disturbing the weather," he said, but added that the weather wasn't so bad as to prevent a successful landing.

"US-Bangla Airlines hasn't been a regular, long-serving airline in Nepali Airport and Nepal has been one of the difficult airports — given the terrain and geography — for landing."

Mr Subedi told ABC that the number of survivors was due to the fast actions of airport firefighters, who put out the flames and freed the injured from the wreckage.

"All the flights to and from have been cancelled at the moment and relief operations are ongoing," he said.

US-Bangla is a private Bangladeshi carrier which operates Boeing 737-800 and smaller Bombardier Dash 8 Q-400 planes.

The airline, part of US-Bangla Group, is based in the Bangladeshi capital, Dhaka, and flies to several domestic and international destinations.

The parent company is involved in a number of industries, including real estate, education and agriculture.

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ABC/wires