WASHINGTON (CNN) -- The United States on Friday pressed for clarification from Iran regarding potentially deal-busting revisions to a nuclear proposal, according to senior Obama administration officials.

The Iranian counter-offer would revise a key element of the draft proposal: the terms of shipping low-enriched uranium for refinement abroad, officials said.

The apparent Iranian rejection of the proposal -- tentatively accepted by Iranian negotiators last week -- centers on Iran not wanting to send its uranium to Russia as agreed on earlier this month at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna, Austria.

Tehran now proposes that its uranium be enriched on Iranian soil by a third country, under IAEA supervision, officials said. Iran said another possibility would be sending out the uranium in several shipments, not in one bulk stockpile.

The officials said the Iranians also want to have talks with the United States about playing a role to ensure the technical safety of the reactor, presumably a way to draw the White House into bilateral talks.

The U.S. officials did not regard Iran's counter-offer as formal, because the Iranian delegate in Vienna delivered it orally.

Speaking in Pakistan, U.S. Secretary of State Hillary Clinton said leading powers that have been involved in the negotiations -- Britain, China, France, Germany, Russia and the United States -- should refrain from immediately judging the status of the nuclear talks.

Clinton told CNN that it was too early to talk about sanctions against Iran.

"I am going to let the process play out, but clearly we are working to determine exactly what they are willing to do -- whether this was an initial response that is an end response or whether it is the beginning of getting to where we expect them to end up," she said.

Iran tentatively accepted the nuclear deal last week after three days of talks with the leading nations, but asked for more time in Tehran to review the document. It sent its response Thursday to the IAEA, the U.N. nuclear watchdog.

The agreement was designed to ease concerns about Iran's nuclear capabilities by increasing transparency of its nuclear program and reducing the amount of material Iran has to possibly make a nuclear bomb.

The United States and its allies fear that Tehran's goal is to make a bomb, which Iran denies.

But the Islamic republic stunned the world in September by revealing the existence of a previously unknown nuclear plant near the city of Qom.

The IAEA this month sent a team of inspectors for a four-day visit to that facility and Iran's state-run Press TV reported that the inspectors "have expressed satisfaction with their mission."

Iranian President Mahmoud Ahmadinejad, addressing a crowd in the northeastern city of Mashhad on Thursday, did not mention Iran's counter-offer, but characterized the draft proposal as a "victory" for Iran.

The agreement acknowledged Iran's right to produce and use low-enriched uranium in violation of U.N. Security Council mandates. However, the industrialized powers signed on because the uranium would be returned from Russia in fuel-rod form designed for civilian purposes.

Ahmadinejad struck a conciliatory note in his speech, saying the West had laid the groundwork for Iran's cooperation in nuclear talks, according to the semi-official Fars News Agency. But he said Tehran was at an important juncture in its nuclear work and that it would not retreat in the least from its nuclear rights.

"We have nuclear contracts. It has been 30 years," Ahmadinejad said. "We have paid for them … such agreements must be fulfilled … for technical activities, for reactors and power plants. If we intend to cooperate, such contracts must be addressed and the previous commitments must be fulfilled."

-- CNN's Elise Labott and Jill Dougherty contributed to this report.

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