No group has so far claimed responsibility for the killings in the relatively tranquil Balkh province, but the attack comes as the Taliban intensify their annual spring offensive despite repeated government attempts to reopen peace talks.
"Those killed in Zari district of Balkh province include seven aid workers - six men and one woman - and two guards," deputy provincial police chief Abdul Razaq Qaderi told AFP, blaming the Taliban for the attack.
"Unfortunately last night unknown armed men attacked our guesthouse in Zari and martyred nine of our employees," a PIN official told AFP, requesting anonymity.
Employees of international aid organisations have increasingly come under attack in Afghanistan despite the Taliban espousing an official policy that rejects attacks on humanitarian workers.
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The latest attack comes just weeks after 14 people - mostly foreigners - were killed in a Taliban attack on a guesthouse in downtown Kabul popular with international aid workers.
In April, the bullet-ridden bodies of five Afghan workers for Save the Children were found after they were abducted by armed gunmen in the strife-torn southern province of Uruzgan.
"Afghanistan is one of the most challenging places for the humanitarian community with death, kidnappings and attacks against humanitarians all too frequent an occurrence," the UN's Afghanistan coordinator Mark Bowden said after that attack.
"I call on all parties to ensure that those providing humanitarian assistance have safe access to people in need and can carry out their life-saving work unhindered."
Official efforts to bring the Taliban to the negotiating table have so far borne little fruit.