The UK has asked Nigeria to explain where and how a Nigerian separatist leader, who holds British citizenship, was arrested.
Nnamdi Kanu - the leader of the group Indigenous People of Biafra (Ipob) - was on the run outside Nigeria for four years until he was brought to court in the capital Abuja last month to face several charges including treason.
His lawyer says he was detained and mistreated in Kenya before being extradited.
A junior minister in Britain's Foreign Office, Tariq Ahmad, said the UK government was seeking clarification from the Nigerian government about the circumstances of Mr Kanu's arrest and detention.
Meanwhile, colleagues of a Yoruba separatist leader - known as Sunday Igboho - say he has been arrested in neighbouring Benin.