A United Nations helicopter crashed while flying in bad weather in Nepal's mountainous east, killing seven U.N. staff and three crew, U.N. officials said.

Secretary-General Ban Ki-moon expressed "great sorrow" after learning of Monday's crash.

There were four U.N. arms monitors on board — from South Korea, Indonesia, Gambia and Sweden — three U.N. staff members from Nepal, and a three-member Russian crew, U.N. spokeswoman Michele Montas said at U.N. headquarters in New York.

The helicopter went down about 200 kilometers (125 miles) east of Kathmandu as it was returning to the capital from a Maoist cantonment site in eastern Nepal's mountainous region, U.N. officials said. Nepalese authorities said the helicopter was flying in bad weather.

Villagers reported seeing a ball of fire after the crash. Nepalese authorities were unable to send rescue helicopters to the area due to the rain and darkness and the only rescue units were on foot, police official Chinu Acharya said by telephone.

Mohan Adhikari, chief of the air rescue unit at Katmandu airport, said rescuers had pulled 10 bodies from the wreckage.

Earlier, the Russian Transport Ministry said the craft was a Russian-made Mi-8 and was carrying eight passengers and three crew members — two Russians and a Belarusian. The discrepancy could not immediately be explained.

Sudden rain on Monday had forced the cancellation of many scheduled commercial flights. Flying in bad weather is often difficult in Nepal's mountainous terrain. Pilots often use their vision to navigate through the mountains and depend less on instruments.

The United Nations has been helping Nepal's peace process since 2006, when communist rebels gave up their armed revolt and joined mainstream politics. The rebels confined their combatants and weapons in camps under the supervision of U.N. arms monitors.