VIENNA (AP) — Renewed failure by Iran and the U.S. to reach a nuclear agreement by a Monday deadline reflects the difficulties each side has with crossing red lines they brought with them to the negotiating table.

With the two sides so far apart when they started the latest round of talks in February, sizable differences remain as talks were extended for another seven months. Here's a look at where things stand:

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URANIUM ENRICHMENT

Both sides agree that Iran should have an "enrichment program with practical limits and transparency measures" to ensure it's peaceful. Uranium can be enriched from low, reactor-fuel level, up to grades used to build the core of a nuclear weapon.

Iran hasn't publicly pulled back from expanding enrichment to a level that would require about 190,000 centrifuges — something the U.S. considers unacceptable. It now has about 20,000 centrifuges, half of them running and is ready to reduce the operational machines to around 7,000 machines. Washington has moved from demanding less than 2,000 to accepting around 4,000.

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FORDO AND ARAK

Because the underground enrichment plant near the Iranian village of Fordo is heavily fortified against aerial attacks, the U.S. and its allies want it shuttered or turned to other uses. Iran wants to keep centrifuges running there even if they aren't enriching — something Washington rejects.

The reactor under construction near the city of Arak is a heavy-water unit that would produce substantial amounts of plutonium that can be used as the fissile core of a missile. The U.S. seeks a completely new kind of reactor that produces only minuscule amounts of plutonium. The Iranians would rather re-engineer it to produce less plutonium — but that process is reversible.

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AGREEMENT LENGTH

An interim agreement says that if Iran honors a final agreement, it will eventually be treated as any other non-nuclear weapons member of the Nuclear Non-Proliferation Treaty. This means Iran would have the right to expand enrichment without having to worry about strict monitoring.

Washington has moved from wanting restrictions over at least 20 years to accepting between 10 and 15 years. Iran wants less than 10 years.

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SANCTIONS RELIEF

Iran wants immediate and permanent relief from U.S., EU and U.N. sanctions, but is unlikely to receive more than a token addition to the easing of some penalties that it has already seen until there is a final deal and a mechanism to ensure its compliance.

The U.S. and its partners have made clear that any lifting of sanctions will be reversible in the case of non-compliance. In Washington, members of the new Republican-majority Congress that will start work in January have already signaled they will ramp up sanctions in the event of an agreement that doesn't dismantle the enrichment process.

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SUSPECTED NUCLEAR WEAPONS WORK

Iran denies wanting — or ever working on — nuclear weapons and has pledged to cooperate with the latest effort by the U.N. atomic agency to investigate such allegations.

But months into the inquiry, Tehran has yet to provide information sought by the International Atomic Energy Agency. While the investigation is separate from the talks, the U.S. says a deal can be struck only if the IAEA is satisfied with the probe and its final results.

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AP Diplomatic Writer Matthew Lee, and Margaret Childs, contributed to this report.