The UN's special torture rapporteur called on the US Tuesday to pursue former president George W Bush and defence secretary Donald Rumsfeld for torture and bad treatment of Guantanamo prisoners.

"Judicially speaking, the United States has a clear obligation" to bring proceedings against Bush and Rumsfeld, the United Nations Special Rapporteur on Torture Manfred Nowak said, in remarks to be broadcast on Germany's ZDF television Tuesday evening.

He noted Washington had ratified the UN convention on torture which required "all means, particularly penal law" to be used to bring proceedings against those violating it.