U.S. Secretary of State John Kerry joined the Geneva negotiations over Iran's nuclear program on Monday as a two-phase deal between Washington and Tehran seemed to be taking shape.

The Associated Press reported the agreement would apparently impose tough restrictions on Iran's nuclear program for at least a decade before providing it some leeway. Tehran would be rewarded for good behavior since last year's agreement when constraints on its uranium enrichment program would be gradually lifted. The crippling economic sanctions imposed on the Islamic Republic would also be eased.

Iran insists it is not interested in nuclear weapons but needs enrichment for energy, medical and scientific purposes.

Under the proposed agreement, the United States wants tough restrictions to last for up to 20 years, though Tehran is pushing for less than a decade.The final deal may end up somewhere in the middle, the news service speculated.

The Islamic Republic could be allowed to operate significantly more centrifuges than the United States first demanded, but they would run at lower capacity than they currently do, the AP detailed. The country would be forced to ship out most of the enriched uranium or change it to a form that is difficult to reconvert for weapons use.

As Kerry and Iranian Foreign Minister Mohammad Javad Zarif completed their two-day talks, negotiators from the parties involved agreed to resume discussions next Monday at a venue to be decided, according to Reuters. Apart from Washington and Tehran, representatives from the remaining four permanent members of the U.N. Security Council, Germany and the European Union are taking part in the talks.

In comments made to the Iranian news agency Fars, Zarif showed himself cautiously optimistic that a deal could be struck before the end-of-March deadline for a framework deal.

"We had serious talks with the representatives (of the permanent members of the Security Council and Germany), and especially with the Americans in the past three days," the foreign minister noted. "But still there is a long way to reach a final agreement."

Kerry, meanwhile, was headed back to Washington on Monday night, the AP noted.