British authorities funded a four-year-long intelligence operation in Nepal that led to Maoist rebels being arrested, tortured and killed during the country's civil war, according to the author of a new book on Kathmandu.

Launched in 2002, "Operation Mustang" targeted Maoist guerillas and saw British intelligence agency MI6 fund safe houses and provide training in surveillance and counter-insurgency tactics to Nepal's army and spy agency, the National Investigation Department, writer Thomas Bell told AFP Saturday.

Nepal's decade-long civil war left more than 16,000 dead, with rebels and security forces accused of serious human rights violations including killings, rapes, torture and disappearances.

"According to senior Nepalese intelligence and army officials involved in the operation, British aid greatly strengthened their performance and led to about 100 arrests," said Bell, whose book "Kathmandu" hits stores in South Asia on Thursday. Many of them were tourtured and killed, he added.