By Angus McDowall RIYADH (Reuters) - Saudi King Salman's meeting on Friday with Hamas leader Khaled Meshaal was part of an effort by Riyadh to shore up Arab unity in the face of a perceived threat from Iran that it believes has become more urgent since last week's nuclear deal. Saudi Arabia, a conservative Sunni Muslim monarchy, has grown more aggressive this year in countering Shi'ite, revolutionary Iran across the region, believing its influence to be the main cause of instability and insecurity in Arab states. Riyadh fears the agreement Tehran struck with six world powers to release it from sanctions in return for more intrusive inspections of its atomic facilities will free Iran to increase its support for Saudi Arabia's enemies in Arab countries.