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

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

Family members of plane crash victims mourn at a hospital in Kathmandu. Narendra Shrestha / EPA

"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, general manager of Tribhuvan International Airport.

But a recording of the conversations between the pilot and air traffic controllers indicated confusion over which direction the plane should land.

In the recording, posted by the air traffic monitoring website LiveATC, conversation veers repeatedly about whether the pilot should land on the airport's single runway from the south or the north.\

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

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

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 Brig. Gen. Gokul Bhandari, the Nepal Army spokesman, said it was clear that at least 50 people had died. Officials at Kathmandu Medical College, the closest hospital to the airport, said they were treating 16 survivors.\

Flight BS211, from Dhaka, the Bangladeshi capital, to Kathmandu was carrying 67 passengers and four crew members, according to a spokesman for US-Bangla Airlines.

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An AP journalist who arrived at the scene soon after the crash saw the twin-propeller plane broken into several large pieces, with dozens of firefighters and rescue workers clustered around the wreckage in a grassy field near the runway. Hundreds of people stood on a nearby hill, staring down at what remained of the Bombardier Dash 8 aircraft.

The plane swerved repeatedly as it prepared to land in Kathmandu, said Amanda Summers, an American working in Nepal. The crowded city sits in a valley in the Himalayan foothills.

"It was flying so low I thought it was going to run into the mountains," said Summers, who watched the crash from the terrace of her home office, not far from the airport. "All of a sudden there was a blast and then another blast."

Fire crews put out the flames quickly, perhaps within a minute, she said, though for a time clouds of thick, darksmoke rose into the sky above the city.

Rescue teams search for survivors in the wreckage of the Bombardier Dash 8. Narendra Shrestha / EPA

The plane had circled Tribhuvan International Airport twice as it waited for clearance to land, Mohammed Selim, the airline's manager in Kathmandu, told Dhaka-based Somoy TV by telephone. The plane was 17 years old, company officials said.

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

"It was flying very low," said 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.

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

Bashanta Bohara, a survivor of the crash, is treated at the Norvic Hospital in Kathmandu. Narendra Shrestha / EPA

Haran Saran was at the hospital hoping for news about his nephew, a medical student.

"He's not on the list of injured people," said Saran, who did not want to give his nephew's name. "We still have hope that there has been some mistake on the list, or he is in some other hospital."

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 did not provide the nationalities of the four crew members.

US-Bangla operates Boeing 737-800 and smaller Bombardier Dash 8 Q-400 planes.

The full-service airline is based in Dhaka and flies to several domestic and international destinations. The parent company, part of US-Bangla Group, is involved in sectors that include real estate, education and agriculture.

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 takeoff, killing all 19 on board.