Looters from rebel Tigrayan forces held aid staff at gunpoint in the town of Kombolcha, the United Nations said.
They stole large quantities of essential food supplies – including some for malnourished children.
Northern Ethiopia is facing mass starvation amid an ongoing civil war between Tigrayan and government forces.
After more than a year of fighting, more than nine million people are in need of critical food supplies in the Tigray, Amhara and Afar regions, the UN says.
A spokesman for the UN, which runs the WFP, said its staff there had faced “extreme intimidation” during days of looting in the industrial hub of Kombolcha in Amhara.
He added: “Such harassment of humanitarian staff by armed forces is unacceptable. It undermines the ability of the United Nations and all of our humanitarian partners to deliver assistance when it is most needed.”
The spokesman also accused military personnel of commandeering three WFP humanitarian trucks and using them for their own purposes.
That led to the decision to halt food distribution in Kombolcha and nearby Dessie, two strategic towns in Amhara that sit on the road to the capital Addis Ababa. The Tigrayan rebels have not commented on the allegations that their fighters stole food aid.
The Ethiopian government recently announced that it had recaptured the towns from the Tigray rebels. But the rebels said the army had only recovered areas they had abandoned.