(Adds quotes, background) 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. In the runup to his visit to the kingdom, officials had said Obama would aim to persuade the monarch that Saudi concerns that Washington was slowly disengaging from the Middle East and ** longer listening to its old ally were unfounded. Last year senior Saudi officials warned of a "major shift" away from Washington after bitter disagreements about its response to the "Arab Spring" uprisings, and policy towards Iran and Syria, where Riyadh wants more American support for rebels.