VIENNA, Austria -- 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 The Associated Press shortly after the 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.

Herman Naeckerts, who headed the International Atomic Energy Agency team that visited Iran, said "remaining differences" scuttled attempts to finalize an agreement on how such an investigation should be conducted. He declined to say whether there was progress.

The IAEA wants the probe to be open-ended, something strenuously opposed by Tehran, which denies it wants nuclear weapons and says it is interested in the atom only as an energy source and for research.

With expectations for success low even before the start of the latest negotiating attempt, interest focused on Iran's move to install a new generation of centrifuges at Natanz, its main uranium enriching site southeast of Tehran.

Iran announced the start of installations during the IAEA team's one-day visit Wednesday at about the same time that the diplomat said the group was shown "a small number" of the machines at the site. The diplomat said those centrifuges were ready to be installed. The diplomat, who closely follows Iran's nuclear program, demanded anonymity because his information was confidential.

The new-generation centrifuges can enrich uranium four to five times faster than Iran's present working model. Experts say Iran already has enough enriched uranium for several weapons if it is further enriched.

Any move to enrich faster will rile Israel, which sees Iran's nuclear program as an existential threat and has said it would use all means to stop it from reaching weapons capability. Israeli Prime Minister Benjamin Netanyahu has warned that the world has until this summer -- at the latest -- to keep Iran from building a bomb.

It also is likely to hurt chances of progress at talks in Kazakhstan later this month between Iran and six world powers seeking to blunt Iran's enrichment program. Iran in turns wants an easing of sanctions imposed over its enrichment program before it is ready to reduce it.

The failure of either side to make the initial move has led to a series of failed negotiations. Nonproliferation expert Mark Fitzpatrick said Iran's centrifuge upgrade may be a further signal that it is determined not to blink first.

"Installation of the more efficient centrifuges will probably contribute to Iran's unwillingness to compromise," said Fitzpatrick, a former senior U.S. State Department official now with the International Institute for Strategic Studies. "It bolsters Iran's belief that time is on its side and that the West will eventually have to give in to the pressure of Iran's growing enrichment capacity.

"It's a kind of mirror image of the Western belief that Iran will eventually have to give in to the pressure of sanctions," he said. "The race between centrifuges and sanctions continues apace."

In first announcing plans to update last month, Iran indicated that It could add more than 3,000 of the new-generation centrifuges to the more than 10,000 older models it has at Natanz turning out low-enriched, fuel-grade uranium. About 700 of the old machines at Fordo, another site, are churning out higher-enriched material that is still below -- but just a technical step away -- from weapons-grade uranium. Iran says it needs that higher-enriched level to fuel a research reactor.

Olli Heinonen, the former IAEA deputy director general in charge of Iran, said that the pace of Iran's installation of its older centrifuges "would mean that all 3,000 plus (new) centrifuges could be installed in six to nine months' time," if the assumption was right that Tehran had the material to make the machines.

When Iran announced its intentions last month, Western diplomats downplayed the proclamation's significance, noting Tehran did not say when it would start populating Natanz with the new machines. But signs of an upgrade that has started or is about to are sure to increase international concerns, particularly if the IAEA verifies as expected in a report later this month that officials saw the equipment ready for installation.

In Washington, U.S. Secretary of State John Kerry urged Iran to show flexibility when negotiators meet in Kazakhstan.

"These talks can only make progress if the Iranians come to the table determined to make and discuss real offers and engage in a real dialogue," Kerry told reporters, speaking alongside UN Secretary-General Ban Ki-moon.

Before his meeting with Kerry, Ban expressed hope the Feb. 26 talks with Iran would bring "fruit for progress."

Associated Press writer Bradley Klapper contributed from Washington