* IS pounded again in **rthern Syria * Group presses attack on strategic border town * Turkey says its troops could be used in Syria * Al Qaeda affiliate vows retaliation against U.S., allies (Adds statement by al Qaeda affiliate) By Mariam Karouny and Jonny Hogg BEIRUT/MURSITPINAR, Turkey, Sept 27 (Reuters) - New U.S.-led air strikes against Islamic State fighters failed to stop them from pressing their assault on a strategic Syrian town near the Turkish border on Saturday, hitting it with shell fire for the first time. The U.S. ...