The United States and Iran are shaping the contours of a deal that would initially freeze Tehran's nuclear program but would allow it to slowly ramp up activities that could be used to make nuclear arms over the last years of the agreement's duration.

Officials from some of the six-power talks with Iran said details still needed to be agreed on, with U.S. and Iranian officials meeting Monday for the third straight day ahead of an end-of-March deadline for a framework agreement.  U.S. Secretary of State John Kerry joined the negotiations after arriving Sunday.

A breakthrough was not expected before Kerry returns to Washington later Monday. But Western officials familiar with the talks cited long-awaited progress on some elements that would have to go into a comprehensive deal. They described the discussions as a moving target, however, meaning changes in any one area would have repercussions for other parts of the negotiation.

The idea would be to reward Iran for good behavior over the last years of any agreement, by gradually lifting constraints on its uranium enrichment program imposed as part of a deal that would also would slowly ease sanctions on the Islamic Republic.

Iran says it does not want nuclear arms and needs enrichment only for energy, medical and scientific purposes, but the U.S. fears Tehran could re-engineer the program to its other potential use -- producing the fissile core of a nuclear weapon.

The U.S. initially sought restrictions lasting for up to 20 years; Iran had pushed for less than a decade.

Iran could be allowed to operate significantly more centrifuges than the U.S. administration first demanded, though at lower capacity than they currently run. Several officials spoke of 6,500 centrifuges as a potential point of compromise.

If the sides agree on 15 years, for instance, the strict controls could be in place for 10 years with gradual lifting over five. Possible easing of the controls could see Iran increasing the number of enriching centrifuges back toward the 10,000 or so it now has operating, and increasing the level of enrichment while keeping it well below levels approaching weapons-grade.

The officials represent different countries among the six world powers negotiating with Iran -- the United States, Britain, China, France, Germany and Russia. They spoke on condition of anonymity because they were not authorized to talk publicly about the negotiations.

The U.N. nuclear agency would have responsibility for monitoring, and any deal would depend more on technical safeguards than Iranian goodwill to ensure compliance.

But the accord will have to receive some sort of acceptance from the U.S. Congress to be fully implemented. That is a tough sell given the hostility to any Iranian enrichment from most Republican and many Democratic lawmakers.

For the United States, the goal of the various restrictions is to extend to at least a year the period that Iran would need to surreptitiously "break out" toward nuclear weapons development. Iran wants relief from the various layers of trade, financial and petroleum sanctions crippling its economy.

The Americans are talking about similarly phasing in measures to ease the sanctions burden on Tehran. Several steps would come immediately through executive action by President Barack Obama, the officials said. Other penalties would be suspended, but not lifted, as Iran demonstrates its compliance with its obligations.

A lesser amount of restrictions would stay in place until Congress acts to remove them permanently.

Still unclear is the status of Iran's underground enrichment facility at Fordo and heavy water reactor at Arak, which potentially could produce enough plutonium for several nuclear weapons a year.