DUBAI, May 14 (Reuters) - Iranian Deputy foreign minister Hossein Amir-Abdollahian said on Wednesday that Tehran had **t received a written invitation for its foreign minister to visit Saudi Arabia, but this was on Iran's agenda, state news agency IRNA reported. On Tuesday, Saudi Arabia's foreign Minister, Prince Saud al-Faisal, said Riyadh had invited Iran's foreign minister to visit. Saudi Arabia is a leading backer of rebels fighting Syrian President Bashar al-Assad, who is a close ally of the kingdom's main rival, Shi'ite Muslim power Iran. ...