Ugandan military prosecutors on Monday added a charge of "treachery" - which carries the death penalty - to the list of violations of military law they say were committed by a prominent opposition figure.

Kiiza Besigye, a veteran political foe of President Yoweri Museveni, who has been in power for almost 40 years, was detained in neighbouring Kenya in November.

He was brought back home and charged with illegal possession of firearms and with undermining the East African country's security in a military court, despite being a civilian.

He has been kept in prison in the capital Kampala since then, together with an aide, Obeid Lutale, with whom he was detained and charged.

Besigye's wife, Winnie Byanyima, the executive director of U.N. agency UNAIDS, has said the charges against him are politically motivated. His lawyers have rejected the charges as baseless.

During a court hearing on Monday, a military prosecutor read Besigye and his co-accused the new charge of treachery.

According to a charge sheet seen by Reuters, Besigye and his co-accused possessed intelligence about a plot to undermine national security but "consciously withheld the said vital information from the proper

Besigye's lawyers protested at the extra charge, saying they violated criminal trial procedures.

They also protested at the detention of prominent human rights lawyer Eron Kiiza, who is part of Besigye's defence team.

At Besigye's last court appearance on Jan. 7, Kiiza was sentenced to nine months in prison for alleged contempt of court over an altercation with court orderlies.

London-based human rights group Amnesty International has described Kiiza's detention and jail sentence as outrageous, demanding his release.

Besigye was once an ally and personal physician of Museveni, but the two later fell out. Besigye ran against and lost to Museveni in four presidential elections. He rejected the results of all those votes over alleged irregularities.

Human rights activists have accused Museveni's government of widespread human rights abuses, including torture and arbitrary detention. The government has repeatedly denied allegations of election fraud and rights abuses.