KAMPALA, – Kenya’s government said it was investigating how a prominent Ugandan opposition leader was spirited out of Nairobi this week, amid growing criticism that it had failed to protect foreign dissidents on its soil.

Kizza Besigye, a longtime rival of Ugandan President Yoweri Museveni, disappeared in the Kenyan capital on Saturday. He reappeared on Wednesday at a military court in neighbouring Uganda, where he was charged with offences including the illegal possession of firearms.

Uganda’s government spokesperson said on Wednesday it did not carry out abductions and that arrests abroad were done in collaboration with host countries.

However in a television interview, Korir Sing’oei, principal secretary at Kenya’s foreign ministry, said Besigye’s detention – which he referred to as an abduction – was “not the act of the Kenyan government” and was being investigated by the interior ministry.

The Ugandan court’s charge sheet alleges Besigye was found with a pistol and eight rounds of ammunition in the Riverside neighbourhood of Nairobi, where it claimed he had been seeking support to undermine the security of Uganda’s military.

Besigye’s wife Winnie Byanyima, who heads the United Nations HIV/AIDS agency UNAIDS, said he has not owned a gun in the last 20 years and should be tried in a civilian, not military, court.

The U.N. human rights chief, Volker Turk, called for Besigye’s release. “Such abductions of Ugandan opposition leaders and supporters must stop, as must the deeply concerning practice in Uganda of prosecuting civilians in military courts,” he said in a statement.

The affair has fuelled renewed criticism of Kenya’s record on human rights and international law.

In July, Kenyan authorities deported 36 members of Besigye’s political party to Uganda, where they were charged with terrorism-related offences. Last month, Kenya deported four Turkish refugees to Ankara, drawing criticism from the United Nations.