The two sides had set a July 20 deadline to resolve their differences, although a temporary agreement late last year incorporated the possibility of another six-month extension, which seems likely. The five permanent members of the United Nations Security Council, plus Germany, want a deal that would ensure that Iran’s enrichment capability was so constrained that it could not “break out” quickly to produce enough fissile material for a bomb, whether made from uranium or plutonium. They want intensive inspections and tough restrictions on enrichment to ensure that Iran cannot secretly build a nuclear bomb.

Iran, which has regularly denied that it desires a nuclear weapon, wants economic sanctions against it lifted and recognition that it can continue to enrich uranium for what it asserts are peaceful uses: medicine and electricity. But the six powers want severe restrictions on the number and type of centrifuges Iran can possess — an issue that one American official characterized before this round of talks as “the sticker-shock conversation.”

Mr. Araghchi said that the two sides would have “one or two more rounds of talks in June,” but no date for another round was set in Vienna.

Negotiators have characterized the talks, which have included separate discussions among technical experts, as cordial and workmanlike, with little rhetorical speechmaking. They are carried out in English, so there is no time wasted on translation. But the cordiality of the discussions has not made an agreement that restores the Security Council’s trust in Iran’s peaceful intentions and Iran’s trust in Western aims any easier.

Both sides say they want to reach a deal, so negotiations are expected to intensify as the July deadline approaches. Senior Western diplomats familiar with the talks, speaking anonymously in accordance with diplomatic practice, have said that they are skeptical of success by the deadline, but that neither side wants a breakdown in negotiations — Iran because it is being hit hard by the sanctions, and the six powers because a failure of the talks might lead to a military strike on Iran by Israel or the United States, which have vowed never to allow Iran to obtain a nuclear weapon.