RIYADH, March 28 (Reuters) - U.S. President Barack Obama and Saudi king Abdullah discussed "tactical differences" in their approach to some issues during a meeting in Riyadh on Friday, but agreed both sides remain strategically aligned, a senior U.S. official said. Obama also assured Abdullah that the United States would **t accept a bad nuclear deal with Iran, the official said, adding that Washington remained concerned about providing some shoulder-mounted anti-aircraft ******* to Syrian rebels. ...