WASHINGTON — The United States has ordered nonessential diplomats and their family members at the U.S. Embassy in Beirut to leave Lebanon, a State Department official said Monday, as tensions over Iran rise with the threat of a potentially imminent military strike. The official said a continuous assessment of the regional security environment determined it was “prudent” to draw down the U.S. Embassy’s footprint so that only essential personnel remain at their posts. The official, who spoke on the condition of anonymity because the move had not yet been formally announced, said that it is a temporary measure and that the embassy will remain operational. Lebanon has been the site of numerous Iran-related retaliatory attacks against U.S. facilities, interests and personnel for decades given Tehran's support for and influence with the Hezbollah militant group, which is held responsible for the deadly bombings of the Marine barracks in Beirut in 1983 and an embassy annex in 1984. As such, changes in the staffing status of the embassy in Beirut have often been seen as a bellwether for