In 1998, confronted by the fact that so much of the art stolen by the Nazis during World War II had yet to be returned to its rightful owners, 44 nations agreed to the Washington Principles, a treaty of sorts that committed its signers to making best efforts to return the looted art. But speaking Monday in Berlin at a conference convened to measure progress in that undertaking on the agreement’s 20th anniversary, the man who negotiated the principles on behalf of the United States delivered a blunt rebuke to what he characterized as foot-dragging by five countries.

“We have made giant strides,” said Stuart E. Eizenstat, an adviser to the State Department, “toward achieving the goals of identifying, publicizing, restituting and compensating for some of the looted art, cultural objects and books, and in so doing, providing some small measure of belated justice to some victims of the Holocaust or their heirs.”

But, he continued, “We must candidly confront the unfulfilled promises we solemnly made.”