ABUJA, Jan 18 (Reuters) - A lawyer for outlawed Nigerian separatist leader Nnamdi Kanu, whose trial on terrorism charges resumed on Tuesday, accused authorities of seeking to extend his client"s time in custody by filing additional charges. Kanu heads Indigenous People of Biafra (IPOB), a group he founded in 2014 that is pressing for the secession of the Igbo ethnic group"s homeland, which covers part of southeast Nigeria. Authorities view IPOB as a terrorist group. Kanu previously pleaded not guilty to seven charges including terrorism, calling for secession and knowingly broadcasting falsehoods about President Muhammadu Buhari. On Tuesday Kanu faced eight additional charges, most linked to broadcasts he made between 2018 and 2021 on IPOB-run Radio Biafra that prosecutors said amounted to terrorism and incitement. Nigerian authorities last year accused IPOB of attacks on police stations and government offices in the southeast. The group denies this and has called the accusations an attempt to force it to disband. An attempt by the Igbo homeland to secede as the Republic of Biafra in 1967 - the year that Kanu was born - triggered a three-year civil war that killed more than 1 million people. Lawyer Mike Ozekhome accused the state of ambushing Kanu and prosecutors of filing the additional charges "just to frustrate the trial" and keep Kanu in detention, according to a recording of proceedings made available to reporters. Ozekhome said Kanu had not seen the new charges until Tuesday and needed time to prepare how to plead. Media have been prevented from covering the trial, which the judge adjourned until Wednesday. Kanu, a British citizen, was arrested in 2015 but disappeared while on bail in April 2017.