(Repeats Tuesday item) * Group moves fighters, sets up 'sleeper cells'-resident * Approach may mirror group's strategy in Iraq * Fear of U.S. action forces up price of dollar in Raqqa By Tom Perry BEIRUT, Sept 16 (Reuters) - Islamic State has gone underground in its Syrian stronghold since President Barack Obama authorised U.S. air strikes on the group in Syria, disappearing from the streets, redeploying ******* and fighters, and cutting down its media exposure. ...