A plane flying 19 people towards Mount Everest went down in flames on the outskirts of the Nepalese capital Friday, killing everyone on board including seven Britons and five Chinese, police said.

The twin-propeller Sita Air plane had just taken off from Kathmandu and was headed to the town of Lukla, gateway to the world's highest mountain, when it plunged into the banks of a river near the city's airport around daybreak.

Witnesses described hearing the screams of passengers and seeing flames coming from one of the plane's wings moments before it hit the ground, while airport authorities said the pilot had reported hitting a bird shortly after take off.

"We could hear people inside the aircraft screaming, but we couldn't throw water at the plane to put out the fire because we were scared that the engines were about to explode," Tulasha Pokharel, a 26-year-old housewife who said she one of the first on the scene, told AFP.

Emergency workers lined up the 19 corpses -- which included seven Nepalese along with the Britons and Chinese -- near the smouldering wreckage as they picked through passengers' belongings to identify the dead.

The British group, the youngest of whom was 27 and the eldest 60, were travelling to the Khumbu area, their agency Sherpa Adventures told AFP, and they were due to go on a 16-day trek to three high passes and the Everest Base Camp.

A crowd of thousands quickly gathered around the riverbank less than a kilometre (half a mile) from the airport, with many shocked bystanders clutching prayer beads and wailing in anguish as they surveyed the devastation.

"The pilots seem to have tried to land it safely on the banks of the river but unfortunately the plane caught fire," police spokesman Binod Singh told AFP, adding that the accident occurred at around 6:30 am (0045 GMT).

Although the exact cause of the crash was still unclear, the manager of Tribhuvan International Airport in Kathmandu said the pilot had reported hitting a bird moments before the crash.

"Immediately after the take-off, the air traffic controllers noticed the aircraft making unusual manoeuvres," Ratish Chandra Lal Suman told reporters.

"When the traffic controller asked the pilot about it, he said the plane had struck a bird," he added.

The crash was the sixth fatal air accident in Nepal in the last two years and it raises fresh questions about safety in the impoverished Himalayan country, home to challenging weather, treacherous landing strips and often lax safety standards.

Ninety-five lives have been lost in air accidents in the last two years, according to an AFP tally, with 15 people killed in the latest crash in May when an Agni Air plane carrying Indian pilgrims went down near northern Jomsom airport.

Six people made a miraculous escape from that accident, including a 30-year-old Danish traveller who survived with nothing more than a bruised leg.

"The record on aircraft flying hours is lax," said Toya Dahal, an air safety specialist with the Initiative for Aviation Safety in Nepal, a lobby group promoting air safety.

"Also, the airlines don't conduct routine maintenance," he added, explaining that they also take risks by flying planes during poor weather conditions.

He cast doubt on the idea that a bird strike had brought down the plane.

"This plane with double engines would have landed safely even after it was struck by a bird. If one engine is damaged, another engine can support the aircraft," he told AFP.

"It looks like the pilot, after noticing technical problems, took the best possible decision to force-land the plane."

The crash is the second disaster to hit mountaineers in Nepal this week at the start of the autumn climbing season, which is the peak time for visiting Nepal, which has eight of the world's 14 highest mountains.

On Sunday, at least eight people were killed in an avalanche on Mount Mansalu in northwest Nepal. The search for three other missing climbers was abandoned on Thursday.

The British embassy in Kathmandu and the foreign office in London confirmed that some of the dead on Friday were Britons but would neither confirm how many nor release their identities.

Nepal has a poor road network meaning that large numbers of tourists, pilgrims and professional climbers often rely on the country's 16 domestic airlines and 49 airports to reach remote areas.