VIENNA -- The United States and Iran say a new deadline in nuclear talks could allow them to finally reach a deal.

But Tehran's reluctance to compromise soon may leave U.S. negotiators running out of ideas on how to reduce Iran's capacity to make nuclear arms.

Western diplomats familiar with the talks said Tuesday they have agreed on little more than to keep talking until June 30, after failing to substantially narrow differences by Monday's deadline in Vienna.

Based on information from the diplomats, progress made so far mostly has stemmed from the U.S. and its allies revising positions closer to the minimum of what they may be able to accept. Iran's demands, in contrast, have changed less -- and the country may be digging in as the next round approaches.

While Ayatollah Ali Khamenei, Iran's supreme leader, has backed the nuclear negotiations, he signaled Tuesday his country would stand firm, saying Washington and its European allies will be unable "to bring the Islamic Republic to its knees."

Washington's mantra has long been "no deal is better than a bad deal," and extending the talks has put off a decision on whether to walk away from them rather than give up too much.

But while the U.S. administration may opt for more wiggle room on the size and capacity of Iran's nuclear program, that may not be the case for powerful sceptics of too much U.S. compromise.

Members of the new Republican-controlled U.S. Congress to be sworn in early next year have threatened to impose additional sanctions on Iran and may have enough votes to overturn an expected veto by President Barack Obama. That almost surely would push Tehran away from the table.

Shortly after the extension announcement, Sen. Mark Kirk, an Illinois Republican whose work with Democratic Sen. Bob Menendez of New Jersey on oil sanctions helped drive Iran to the negotiating table, pledged to come forward with a new bipartisan sanctions package.

Menendez suggested similar action, saying he'd work "to ensure that Iran comprehends that we will not ever permit it to become a threshold nuclear state."

Key to an agreement for the U.S. is capping Iran's uranium enrichment program.

Iran denies interest in atomic arms and says it needs to enrich to make reactor fuel and for other peaceful uses.

But the West fears Tehran could ramp up the program and enrich to levels used for the fissile core of nuclear weapons.

Iran has nearly 10,000 centrifuges enriching uranium. Washington came to the negotiating table 10 months ago demanding Tehran pare down that number to fewer than 2,000 in attempts to increase the time Iran would need to make enough weapons-grade uranium from a few months to a year or more.

The U.S. is ready to accept just over 4,000, the diplomats said on condition of anonymity because they are not authorized to discuss the confidential talks. Iran also has moved -- but not nearly as much. It is floating around 8,000 centrifuges as acceptable.

Washington also has compromised on the timeline for the enrichment restrictions. Originally it wanted the caps in place for 20 years or more. Now, it may be ready to accept 15 years.

Iran, however, wants limits substantially below 10 years, while continuing to insist on expanding the program immediately after constraints are lifted. It is aiming for enriched uranium output at that point that would equal nearly 200 percent more than that the centrifuges it now has.

That is unacceptable to the Americans.

With centrifuge numbers in dispute, the U.S. and its six partners at the negotiating table, including Russia and China, have proposed other ways to push Iran's "breakout" time for making enough weapons-grade uranium for a bomb to a year or more.

One way would be to persuade Tehran to ship out much of its stockpile of uranium gas, reducing it from about seven tons to no more than half a ton. Iran appeared interested at least until Wednesday, said the diplomats, negotiating with Russia as the possible recipient.

But in a potential complication last week, Iran's nuclear chief, Ali Akbar Salehi said: "There is no reason to send our fuel to Russia."