(KABUL, Afghanistan) — Afghan troops backed by U.S. airstrikes massed Tuesday on the outskirts of a key northern city seized by the Taliban in the buildup for what is likely to be a long and difficult campaign to drive out the insurgents. Despite the overnight U.S. bombing of Taliban positions, the militants extended their grip on Kunduz, forcing government forces to retreat from a strategic fortress hilltop and giving the insurgents a vantage point overlooking the city.