Uganda threatened Wednesday to penalise media for coverage of a purported plot aimed at stifling allegations that President Yoweri Museveni is grooming his son for power.

Speculation is growing that Museveni, in office since 1986, is lining up his son, Kainerugaba Muhoozi, to succeed him, a move that would test loyalties in Uganda's ruling elite.

Last week, a newspaper published a private letter by General David Sejusa calling for an investigation into claims of a plot "to assassinate people who disagree with this so-called family project of holding onto power in perpetuity."

The letter by Sejusa, one of Uganda's most senior officers and long regarded as close to Museveni, has sparked public debate on the sensitive and rarely aired question of whether Museveni, 68, will step down at the end of his term in 2016.

Godfrey Mutabazi, executive director of the state media regulator, the Uganda Communications Commission (UCC), said newspapers and radio stations had given the letter "undue" attention.

"Why give Sejusa headline after headline? ... On airwaves you hear nothing else except Sejusa," Mutabazi told Reuters.

"No, this obsessive coverage is not proper. They're alarming the public, causing insecurity and destabilising the country and we can't allow that ... We'll penalise this behaviour."

Mutabazi did not specify what penalties the UCC would impose. Ugandan journalists say such warnings often herald harassment by security agencies.

Don Wanyama, managing editor of the Daily Monitor, which published the letter, told Reuters that police detectives had interrogated him and two reporters for over six hours Tuesday.

He said the police had demanded that the journalists hand over an original copy of Sejusa's letter and also disclose how they obtained it. They refused and were threatened with charges, but were later released.

Although authorities in Uganda, a prospective crude oil producer, have traditionally given space to critical media, the police often call in journalists for interrogation after reports on controversial subjects.

Opposition-led protests in 2011 sparked a security crackdown that killed at least nine people. In addition to suppressing political opposition, critics say Museveni has failed to curb corruption, hurting public services and prompting donors to cut aid.