ALMATY, Kazakhstan — Two days of talks between six world powers and Iran over its nuclear program ended on Wednesday with specific agreement for further meetings in March and April over a proposal that would sharply constrain Iran’s stockpile of the most dangerous enriched uranium, in return for a modest lifting of some sanctions.

But the six powers dropped their demand that Iran shut down its enrichment plant at Fordo, built deep underneath a mountain, instead insisting that Iran suspend enrichment work there and agree to take a series of steps that would make it hard to resume producing nuclear fuel quickly. The six also agreed, in another apparent softening, that Iran could keep a small amount of 20 percent enriched uranium — which can be converted to bomb grade with modest additional processing — for use in a reactor to produce medical isotopes.

It was unclear whether any of these new positions would pave the way for any kind of agreement. Iran’s negotiators must now take the new proposal back to Iran’s supreme leader, Ali Khamenei, at a time of intense maneuvering and infighting in Iran. The two sides agreed that technical experts would meet to discuss the proposal on March 18 and 19 in Istanbul, while the negotiations at this higher political level would resume, again in Almaty, on April 5 and 6.

The chief Iranian negotiator, Saeed Jalili, called this week’s meeting positive, asserting at a news conference that the six powers, representing the permanent members of the United Nations Security Council plus Germany, had offered a revised proposal that was “more realistic” and “closer to the Iranian position.”