A defence expert says a US ground invasion into Iran would be high risk and may not achieve its stated goals. 

Donald Trump has told the Financial Times he's considering seizing Iran's major oil terminal on Kharg Island.

Defence Priorities Senior Fellow Gil Barndollar says the island's deep in the Strait of Hormuz and difficult to get to. 

He says Trump probably wants it for leverage. 

"You take the port, and 97 percent of their oil goes out through, as well as whatever amount of oil is there right now, and use that as a bargaining chip."

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