LONDON, July 20 (Reuters) - Britain has asked Nigeria to explain where and how Nnamdi Kanu, a separatist leader who holds British citizenship, was arrested after Kanu"s lawyer alleged he had been detained and mistreated in Kenya before being sent back to Nigeria. Kanu, leader of the Indigenous People of Biafra (IPOB) group which wants part of southeastern Nigeria to secede, was on the run outside Nigeria for four years until he was brought to court in Abuja on June 29 and told he would face trial. read more "We are seeking clarification from the Nigerian government about the circumstances of the arrest and detention of Nnamdi Kanu," said Tariq Ahmad, a junior minister at Britain"s Foreign Office. The Nigerian authorities have refused to say where Kanu was arrested, while Kenya"s ambassador to Nigeria has denied his country was involved. read more Responding to a question about Kanu"s case from a lawmaker, Ahmad said Britain had requested consular access to Kanu and stood ready to provide consular assistance. A spokesman for Nigeria"s Department of State Services, the security agency that is holding Kanu, could not immediately be reached for comment on Tuesday, a public holiday in Nigeria. Kanu"s lawyer, Ifeanyi Ejiofor, alleged he was abducted by Kenya"s special police force and held for eight days before being turned over to Nigerian authorities. read more Kanu faces 11 charges including treason, terrorism and illegal possession of firearms. His case is due to resume in court in Abuja next week. IPOB wants a swathe of the southeast to split from Nigeria. The region attempted to secede in 1967 under the name Republic of Biafra, triggering a three-year civil war in which more than a million people died, mostly from starvation. The Biafran enclave was reabsorbed into Nigeria after the war, but despite an official "no victor no vanquished" policy adopted at the time, discontent has continued to simmer in the region.