Yale University officials blamed different methods of counting artifacts for its discrepancy with Peruvian officials over the number of antiquities the university is holding from the famed Inca citadel of Machu Picchu.

Peru's government and Yale reached an agreement last year to return 4,000 pieces that had been taken from the site a century ago.

But Peruvian Health Minister Hernan Garrido Lecca, who is heading negotiations with Yale, said Sunday that a government-led commission found that the university has 40,000 artifacts.

Yale spokesman Tom Conroy said the difference stems from how the pieces are counted. For example, he said, fragments from one object could be counted as one piece or more based on the number of fragments.

"The difference is how one chooses to count, not what Yale has in its collection from Machu Picchu," Conroy said. "We're talking about the same inventory we shared with them last month."

Peru had demanded the collection back last year, saying it never relinquished ownership when Yale scholar Hiram Bingham III rediscovered Machu Picchu in 1911.

Yale is continuing to pursue a final agreement based on a memorandum of understanding reached last fall, Conroy said. The difference in how the objects are counted is not an obstacle to the final agreement, he said.

When the deal to return the artifacts was announced in September, the New Haven, Connecticut-based university said in a statement that some of the pieces would remain there temporarily for research, but did not specify how many.

Yale and Peru are to co-sponsor a traveling expedition featuring Bingham's pieces and later a museum in the Andean city of Cuzco, the ancient Inca capital, under the deal.

The ruins at Machu Picchu, located on a mountaintop above a lush valley southeast of Lima, are Peru's top tourist attraction.