President Barack Obama has issued his most direct threat yet of US military action against Iran if it builds a nuclear weapon, but in a message to Israel's leader ahead of White House talks he also cautioned against a pre-emptive Israeli strike. Speaking in a magazine interview published on Friday, three days before he will host Benjamin Netanyahu, the Israeli prime minister, in Washington, Mr. Obama said he was not bluff when saying that. With the meeting expected to be dominated by stark differences over what Washington fears could be an Israeli attack on Tehran's nuclear sites, Netanyahu said he wanted to preserve the "freedom of action of the State of Israel in the face of threats to wipe us off the map". Obama repeated the US refrain that "all options are on the table" but spoke in his most direct terms yet of a possible US military response if sanctions and diplomacy fail to curb Tehran's nuclear ambitions.