YENAGOA, Nigeria (Reuters) - A Nigerian separatist leader arrested after years on the run was badly mistreated in detention in Kenya before being returned to Nigeria, his lawyer said on Friday, while the Kenyan high commissioner flatly denied his country's involvement.

       Nnamdi Kanu, leader of the Indigenous People of Biafra (IPOB) group, disappeared from Nigeria after skipping bail in 2017.

       He appeared on Tuesday, handcuffed and hooded, at a court in Abuja, flanked by security agents. Nigerian authorities said he had been captured abroad and brought back to face charges, but refused to say where he had been caught.

       On Friday, Kanu's lawyer, Ifeanyi Ejiofor, said Kanu was abducted by Kenya's special police force and held for eight days before being turned over to Nigerian authorities.

       "He was tortured and subjected to all forms of inhuman treatment which worsened his health condition," Ejiofor said in a statement.

       Wilfred Machage, Kenya's high commissioner to Nigeria, emphatically denied his country's involvement in Kanu's arrest. In a statement posted on Twitter, he called any such charge "fictional, imaginary and deliberately concocted to fuel antagonistic feelings among certain sections of the Nigerian people."

       IPOB's 1967 campaign for an independent nation in southeastern Nigeria, called the Republic of Biafra, triggered a three-year civil war in which more than a million people died, mostly from starvation.

       Kanu is facing 11 charges including treason, terrorism and illegal possession of firearms. His case is due to resume in court on July 26-27.

       (Reporting by Tife Owolabi, writing by Libby George; Editing by Howard Goller)