BOCA RATON (AFP)---US presidential challenger Mitt Romney called Monday for Iranian President Mahmoud Ahmadinejad to be indicted under the UN genocide convention over his comments questioning the Holocaust.

In the last of three debates with President Barack Obama ahead of November 6 elections, the Republican vowed a harder line on Iran.

"I'd make sure that Ahmadinejad is indicted under the Genocide Convention.  His words amount to genocide incitation. I would indict him for it," Romney said.

 "I would also make sure that their diplomats are treated like the pariah they are around the world. The same way we treated the apartheid diplomats of South Africa," Romney said.   

The UN Convention on the Prevention and Punishment of the Crime of Genocide, drafted after the Holocaust, calls for punishment against "direct and public incitement" to commit genocide, along with the act itself.   

The convention was invoked in setting up international tribunals to try abuses in the former Yugoslavia and Rwanda.   

Iran is party to the convention, signing it before the 1979 Islamic revolution overthrew the pro-Western shah

 The convention does not explicitly make it a crime to deny the Holocaust.

Several European countries criminalize Holocaust denial, but the United States offers wide freedom of speech under its Constitution.

Ahmadinejad has repeatedly questioned the Holocaust. At a 2009 rally to support the Palestinians, Ahmadinejad said that "the pretext for establishing the Zionist regime is a lie."