Armed men opened fire at a wedding ceremony in northern Afghanistan, killing at least 21 guests and wounding eight, provincial officials said on Monday. Violence has increased sharply this year in most parts of Afghanistan after U.S.-led forces formally ended their combat mission at the end of 2014, leaving a small contingent of about 12,000 NATO troops to train Afghan forces. It was not immediately clear what prompted the attack in the northern province of Baghlan, but insurgent activity has increased in the province over the past year and private militias have proliferated.