A UN expert on Monday warned that the dismissal of Nigeria’s Chief Justice by President Muhammadu Buhari violated international human rights standards on independence of the judiciary and the separation of powers.

The president of Nigeria suspended Chief Justice Walter Onnoghen on January 25. The president insisted that he had acted in compliance with an order issued two days earlier by a tribunal established under the Nigerian Constitution to decide on alleged breaches of the Code of Conduct for Public Officials.

However, the tribunal had previously said it lacked jurisdiction over cases involving judicial officers, and four higher jurisdiction courts—the Court of Appeal, the National Industrial Court and the two Federal High Courts—had already ordered a stay of proceedings in the Tribunal.

“International human rights standards provide that judges may be dismissed only on serious grounds of misconduct or incompetence,” said Diego Garcia-Sayán, the UN Special Rapporteur on the independence of judges and lawyers. “Any decision to suspend or remove a judge from office should be fair and should be taken by an independent authority such as a judicial council or a court.”

Garcia-Sayán said that one of the senior advocates defending Onnoghen was arrested on Wednesday and others had been “subject to serious threats, pressures and interferences.” He warned that such allegations “may constitute grave attacks to the independence of the judiciary and the free exercise of the legal profession.”