VIENNA — Adding weight to its announcement of a nuclear upgrade, Tehran has shown high-level UN officials high-tech equipment positioned at its main uranium enrichment site meant to vastly accelerate output of material that can be used for both reactor fuel and atomic arms, a senior diplomat said Thursday.

The diplomat spoke to reporters shortly after the UN officials returned from Tehran, acknowledging that their latest in a series of trips to the Iranian capital that began over a year ago again failed to reach a deal to restart an investigation into suspicions that Iran is pursuing nuclear arms.