Donald Trump has criticised Barack Obama for pulling US troops out of Iraq too quickly and claimed that Iran had taken control of the world’s second biggest oil reserves.

Speaking in Ohio ahead of primary elections in five states on Tuesday, Mr Trump claimed that he had always been opposed to the US’s 2003 invasion of Iraq. He said he had warned it would destabilise the Middle East and that the rise of Isis was proof of that.

“Saddam Hussein was not a good person. Who cares,” Mr Trump said, speaking in a private hangar at the city’s airfield.

Mr Trump was introduced by New Jersey Governor Chris Christie (AP)

“He was very good at killing terrorists. Now, Iraq is the Harvard for terrorism.”

Mr Trump, who was introduced to the crowd of around 1,500 by New Jersey Governor Chris Christie, said that Iran had stepped into the vacuum created by the withdrawal of most US combat troops.

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“Iran is taking over Iraq. They’re taking over the second largest oil reserves in the world,” he said.

“I said ‘take the oil’ and they said ‘you can’t do that - it’s a sovereign country’. Sovereign country? These were corrupt people.