And, frankly, the US is flatyly denying it, including the person is supposed to have said it. He's been shown a cable that was supposedly issued by his office, and which he is rejecting as his words.
The phrase "bomb [you] back to the Stone Age" actually dates from the Vietnam area and was famously made in discussion about Cambodia and their unwillingness/inability to prevent the Viet Cong from slipping back and forth across the Vietnamese/Cambodian border. So I find it highly suspicious that those precise words would be used again after the political fallout they caused in 1968.