Nepalese army colonel Kumar Lama, currently serving as a UN peacekeeper in South Sudan, has appeared in a London court charged with two counts of torture.
The 46-year-old stands accused of inflicting severe pain or suffering on two men when he was in charge of a barracks during the Himalayan nation's decade-long Maoist insurgency in 2005.
Lama spoke only to confirm his identity when he appeared for a short hearing at Westminster Magistrates' Court on Saturday.
His arrest in the UK has caused a spat between London and Kathmandu, with the Nepali government demanding his immediate release.
Nepal summoned the British ambassador on Friday to express its "strong objection" to Lama's detention.
The colonel spent a Christmas break in England with his family. He was arrested under British law, which allows prosecutors to act against people suspected of torture no matter where it took place in the world.
Rights groups accuse both the security forces and former Maoist rebels of committing abuses including torture during the conflict that killed more than 16,000 people.
The Maoists ended the conflict in 2006 under a peace deal with the government, won elections four years ago and are now heading a coalition ruling the republic.
A police statement accused Lama of intentionally inflicting "severe pain or suffering" on Janak Bahadur Raut between April 15 and May 1, 2005, and on Karam Hussain between April 15 and October 31, 2005.
Human Rights Watch said the arrest sent a warning to those accused of serious crimes that they cannot hide from the law.
"The UK's move to arrest a Nepali army officer for torture during Nepal's brutal civil war is an important step in enforcing the UN Convention against Torture," Brad Adams, Asia director of Human Rights Watch, said in a statement.