WASHINGTON/JERUSALEM (Reuters) - In a highly unusual rebuff to a close ally as tensions escalated over how to deal with Iran's nuclear program, the White House said on Tuesday President Barack Obama would not meet Benjamin Netanyahu during the Israeli prime minister's U.S. visit later this month. The apparent snub, coupled with Netanyahu's sharpened demands for a tougher U.S. line against Iran, threatened to plunge U.S.-Israeli relations into crisis and add pressure on Obama in the final stretch of a tight presidential election campaign.