KABUL (REUTERS) - Armed men opened fire at a wedding ceremony in northern Afghanistan, killing at least 21 guests and wounding eight, provincial officials said on Monday (July 27).

Violence has increased sharply this year in most parts of Afghanistan, after US-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.

"There was a wedding party, which turned to a tragedy, after some armed men entered and opened fire on male guests," Baghlan police chief General Abdul Jabar Pordeli told Reuters by telephone. "An investigation is ongoing and police will arrest all perpetrators," added the police chief, who estimated there were 2,500 armed members of illegal groups in the province.

Last year was the deadliest on record for non-combatants in Afghanistan, the United Nations has said, with at least 3,188 Afghan civilians killed in the intensifying war with Taliban insurgents.

Weddings in rural Afghanistan start in the evening and parties follow a custom of keeping men and women guests apart.