When the Metropolitan Museum of Art struck an accord with Italy last month to return antiquities in exchange for long-term loans, both sides said they hoped the agreement would serve as a model for future disputes between nations and museums.

Many cultural observers saw the pact as beneficial both for Italy and the Met. Now, as American museums are facing increased claims against objects in their collections, one leading diplomat is rejecting that framework as an unacceptable compromise.

The antiquities chief of Egypt, Zahi Hawass, said in an interview this week that he would never have made the agreement that his Italian counterpart, Cultural Minister Rocco Buttiglione, struck with the Met.

“I would not make deals,” Mr. Hawass, the secretary-general of Egypt’s Supreme Council of Antiquities, said shortly before delivering a lecture at the Met on Wednesday night.

Mr. Hawass is taking his hard-line stance on cultural property to the St. Louis Art Museum. He has demanded the return of a 3,200-year-old mummy mask that the museum bought in 1998. Although the museum says it did extensive research on the origin of the mask before purchasing it, Mr. Hawass contends the artifact disappeared from the Egyptian Museum in Cairo.

He says he has sent proof to the museum and is threatening to take “a big legal action” if the museum does not agree to return the mask, which features a bust of a young woman known as Ka Nefer Nefer. “The director of this museum is still playing with us,” Mr. Hawass said. “I will make this museum as a criminal, to be listed as a criminal museum.”

A spokeswoman for the museum, Jennifer Stoffel, said officials were examining the evidence and that the museum would respond when its director, Brent Benjamin, returned from a trip abroad. Mr. Benjamin has previously said in a statement that the museum was not aware of any credible information suggesting the piece was stolen, but that it was prepared to “further investigate the claim” and looked forward to working with Mr. Hawass.

The claim is one of several being lodged against museums across the country. Having signed its agreement with the Met, Italy is now in talks with the J. Paul Getty Museum in Los Angeles and is planning to meet with officials from the Princeton University Art Museum and the Museum of Fine Arts in Boston about pieces in their collections. The Getty Museum said yesterday that it also was “in the process of establishing a dialogue” with Greece about the provenance of objects in its collection.

A spokeswoman for Princeton, Cass Cliatt, said museum officials would discuss four pieces in its collection early next month in Rome. The MFA’s deputy director for curating, Katherine Getchell, said the museum had not received a list of disputed objects from Italy. A meeting scheduled for next month in Rome was postponed this week, she said, when Italian officials requested more time to prepare. Although it is early in the process, Italy’s arrangement with the Met could figure in the MFA’s talks. “The Met agreement is interesting and useful to us as we go into this dialogue,” Ms. Getchell said.

Italy’s claims against the American museums stem from evidence it has collected in investigating an Italian art dealer, Giacomo Medici, an American art dealer, Robert Hecht, and a former Getty curator, Marion True, on charges of trafficking in looted art. Medici has been convicted, while the trial of Mr. Hecht and Ms. True is under way in Rome. Both deny the charges.

The investigation and claims have spawned a debate in American art circles over the ownership of cultural property and whether current laws and import restrictions are the most effective way to prevent looting at ancient cultural sites across the globe. To Mr. Hawass, the answer is simple, if extreme: Museums, he said, should stop buying antiquities altogether.

Museum directors, however, defend their role in collecting antiquities as promoting knowledge and cultural scholarship. “I don’t think there’s any evidence that that would work,” the director of the Art Institute of Chicago, James Cuno, said of Mr. Hawass’s suggestion. “It would drive these objects underground to the black market. If we don’t collect these things, someone else will,” he said.