The Agni Air plane was returning to Kathmandu after poor visibility prevented it from landing at Lukla, its intended destination in a popular trekking spot in eastern Nepal, government officials said.

Rescuers at the scene of the crash described it as "horrific", with the badly burned remains of the victims strewn over a field around 15 miles (25 kilometres) south of Kathmandu.

The remains were flown by helicopter to Kathmandu airport, where they were met by the US and Japanese ambassadors and by caretaker prime minister Madhav Kumar Nepal, who said he was deeply saddened by the tragedy.

The cause of the crash was not immediately clear and Bimlesh Lal Karna, head of rescue operations with the civil aviation authority, said an investigation had been launched.

"A committee has been formed and it will report on its findings within 10 days," said Karna, who suggested the weather conditions may have been the reason for the accident.

Witnesses said they saw the plane crash into a field next to a school in heavy rain and break up on impact.

"There are small pieces of the plane all over the field and you can see body parts. We are all so shocked," villager Pratap Lama told the Kantipur radio station.

Bishnu Adhikari, managing director of the travel agency that booked the six foreigners on to the flight, said they were all planning to trek to Everest base camp.

"The Americans were travelling together and they had a guide with them. The British guy also had a guide, and there was a young Japanese man who was travelling alone," said Adhikari, of Himal Reisen Travel.

"They were all very excited by the prospect of seeing Everest for the first time. This is a very sad time for us all."

Thousands of travellers fly into Lukla, 140 kilometres (90 miles) northeast of Kathmandu, every year to access the stunning Himalayan range that forms Nepal's northern border with Chinese-controlled Tibet.

The 550-metre-long (1,800 feet) sloping airstrip, perched on a hillside 2,757 metres above sea level, is considered one of the most difficult landings in the world and there have been several accidents there.

The last was in 2008 when a Twin Otter plane carrying 18 people crashed, killing everyone on board.

The airport is used by climbers heading for the heights of Everest, though August is the low season for both mountaineering and trekking.

August is the rainy season in Nepal and helicopters were initially unable to fly to the crash area due to poor visibility. Landslides -- a frequent occurrence in Nepal during the monsoon -- also hampered road access.

Tourism is a major foreign currency earner for impoverished Nepal and the number of visitors has increased since a civil war between Maoist guerrillas and the state ended in 2006.