The announcement follows a similar one by the U.S. Agency for International Development, which said on Wednesday it had discovered that food intended for people in Tigray, millions of whom are in severe need of aid, was being diverted and sold.
Neither organization gave details of the source of the reports and the WFP did not say who was responsible for the diversions or when they had taken place.
But it said late on Wednesday it was "strongly reiterating to our cooperating partners that they monitor and report any illicit activities, and that they are enforcing the agreed controls."
Getachew Reda, the head of Tigray's interim regional government, urged humanitarian agencies to reconsider suspending aid, saying in a statement that the moves would "hurt our people who are facing grave challenges."
He said he had set up a task force to investigate, calling the reported theft a crime against children, the elderly and the disabled.
A spokesperson for Ethiopia's federal government did not immediately respond to a request for comment.
The government and Tigray forces agreed to end hostilities in November, which has allowed additional aid to reach the region and for some services to be restored.