"I have exact information that Bin Laden was held by the American military for sometime... until the day they killed him he was a prisoner held by them," the hardline president said in a live interview on Iranian state television.
"Please pay attention. This is important. He was held by them for sometime. They made him sick and while he was sick they killed him," Ahmadinejad added.
He accused US President Barack Obama for announcing the al Qaeda leader's death for "political gain."
"What the US president has done is for domestic political gain. In other words, they killed him for Mr Obama's election and now they are seeking to replace him with someone else," Ahmadinejad said without elaborating.
Bin Laden was shot dead on May 2 in a US commando raid on a heavily fortified compound near Islamabad, Pakistan.
On May 4, Iranian Defence Minister Ahmad Vahidi too had cast doubt on Bin Laden's death, saying there were "ambiguities" over the way he was killed.
The Americans "said they threw his body in the sea. Why did they not allowed an independent expert to examine the body to say if it was Bin Laden or not?" Vahidi said.
Iran has always considered al Qaeda as a threat to its security. But the Islamic republic has regularly criticised the presence of US-led Western forces in Afghanistan, saying it prevented any negotiated settlement of the Afghan conflict.
Tehran also regularly denounces the presence of US forces in Arab states of the Gulf, calling it a source of regional tension.
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