WASHINGTON (Reuters) - As the Taliban kicks off its spring fighting season in Afghanistan, an agreement with Pakistan that would help NATO supply its troops there could be weeks or months away, forcing military leaders to spend two-and-a-half times as much to ship some supplies through Central Asia. The Obama administration remains locked in negotiations with Pakistan to reopen the key supply routes into Afghanistan, and officials do not expect talks bogged down over proposed tariffs and U.S. military assistance to reach resolution anytime soon. ...