BRUSSELS — European Union countries must purchase military equipment made in Europe under a new loan program meant to help the continent provide for its own security, a top EU official said Tuesday, even though most of its defense materiel currently comes from U.S. suppliers. At a summit last week , the EU’s 27 leaders weighed a European Commission proposal for a new loan plan worth 150 billion euros ($163 billion). It would be used to buy air defense systems, drones and “strategic enablers”