SIRTE, Libya — The black flag of the Islamic State flutters above a two-story house that serves as a sniper’s nest. The deserted street is under their control, as are all the nearby buildings. The banner and the occasional crack of the marksman’s bullet are the only signs of the militants on this afternoon.Two blocks north, less than a football field away, Libyan militiamen are gathered with hand-held rocket launchers and Mad Max-style pickup trucks mounted with large machine guns. Since Monday, U.S. airstrikes have pounded Islamic State targets in this battered seaside city, the stronghold of the Middle Eastern militants’ Libyan affiliate. Yet the pro-government militia forces have not crossed this front line.