President Donald Trump warned on Wednesday that US forces would strike Iran "hard" in the coming night, after earlier declaring Washington's ceasefire with Iran over at a NATO summit in Ankara.

"We're gonna hit 'em hard tonight," he said before holding talks with Ukraine's President Volodymyr Zelenskyy, saying: "They violate the agreement every day."

Trump also threatened to strike Iran’s civilian infrastructure and to seize Kharg Island, which is home to extensive oil infrastructure.

“Maybe we’ll take over Kharg Island. We may take over Kharg Island. There’s not a thing they could do about it,” Trump said.

Trump had said that the ceasefire with Iran is "over" after Washington launched strikes on the country overnight, to which Iran responded by targeting US bases in the Gulf.