Iranian authorities hanged a man convicted of drug offences even though he had already been killed in a confrontation with prison officers just prior to his planned execution, an NGO said Monday.

The man, named as Nematullah Barahui, was to be hanged with another man also convicted of drug offences in Zahedan prison in southeastern Sistan-Baluchistan province on Sunday, according to Iran Human Rights (IHR), a Norway based organisation.

But it cited unspecified local sources saying that he was killed in an altercation with officers before the planned execution, and that the authorities hanged him anyway to prevent any legal problems for the prison guards.