(Adds Saudi ambassador in Leba**n on Beirut bombing) RIYADH, June 26 (Reuters) - king Abdullah ordered all necessary measures to protect Saudi Arabia against potential "terrorist threats" after chairing a security meeting to discuss the fall-out from Iraq, the state news agency SPA said on Thursday. The world's top oil exporter shares an 800-km (500-mile) border with Iraq, where the militant Islamic State in Iraq and the Levant (ISIL) and other Sunni Muslim groups have seized towns and cities in a lightning advance this month.