The United States says it's troubled by reports implicating Burundian security forces in the extra-judicial killings of at least 24 members of a rebel group who surrendered last month in the East African nation.

The U.S. State Department, in a statement late Thursday, also cited the alleged increase in irregular detentions and prosecutions of journalists and members of political parties ahead of elections in May.

In response, Pierre Nkurikiye, a spokesman for Burundi's police, denied that any rebels were killed but declined to speak about the allegations against the security forces.

Human Rights Watch says Burundi's government has persistently harassed and blocked the activities of opposition parties, civic groups and the press.