President Shimon Peres said on Friday that Israel was not responsible for the assassination of a nuclear scientist in Tehran earlier this week.

In an interview with CNN’s Spanish-language channel, Peres said, “To my knowledge, Israel is not behind the assassination. I am well aware that it is ‘fashionable’ to blame Israel and the U.S. whenever something negative happens there.”

Peres’ comments came one day after U.S. President Barack Obama called Israeli Prime Minister Binyamin Netanyahu on the phone and reportedly wanted explanations for the assassination.

Obama reportedly demanded explanations from Netanyahu following reports in the press as well as accusations by Iran that Israel was involved in the killing, and also sought to make clear to Netanyahu American expectations from Israel regarding Iran. The U.S. reportedly wants Israel to refrain from attacking Iran and to coordinate all Iran-related actions with Washington.

In his interview with CNN, Peres called on South American leaders not to host Iranian President Mahmoud Ahmadinejad, who is currently on an official tour of the region. Peres said Ahmadinejad is “the most corrupt leader in the world” and added that South American citizens are “peaceful people” and, as such, Ahmadinejad should be declared persona non grata in the region.

On Thursday, Ahmadinejad said his country was not looking to develop a nuclear weapon. Speaking to reporters in Cuba, the Iranian President said, “The nuclear question is a political excuse. They know that Iran is not looking to make atomic bombs. Iran is not as imprudent as they are.”

He added, “We do not believe in making atomic bombs. We believe that goes against human morality.”