It is a sunny day in Tehran. To soothing piano music, an avuncular figure walks out the door of a building studded with Persian mosaics, past a burbling fountain and into a courtyard. Facing the camera, he says: “In the next three weeks we have a unique opportunity to make history.”

Iranian Foreign Minister Mohammad Javad Zarif’s four-minute video, posted Wednesday on YouTube, signaled his government’s intention to claim credit if negotiations over its nuclear program have succeeded when they reach their scheduled end on July 20, or to direct blame elsewhere if they have failed.

Secretary of State John F. Kerry has already made a similar case for the international coalition on the other side. “The United States and our partners have demonstrated to Iran how serious we are,” Kerry wrote in a Washington Post op-ed Monday. “Now Iran must choose.”

The final round in the months-long negotiations opened Wednesday in Vienna, where Zarif met with Kerry’s deputy, William J. Burns, and European Union foreign policy chief Catherine Ashton. Talks are expected to continue for the next three weeks without pause.

The specifics remain secret, but U.S. officials and nuclear experts expressed little optimism about a deal. Participants are said to remain far apart on key elements — a reduction in Iran’s uranium enrichment capability and elimination of a plutonium-producing reactor, full transparency for Iranian nuclear activities and the end of Western sanctions.

“From the U.S. perspective, the key thing is that this isn’t a negotiation among parties with equal standing, trying to find a middle ground. This is more about compliance, so there are some absolute minimal conditions that aren’t negotiable,” said George Perkovich, a nonproliferation expert and vice president for studies at the Carnegie Endowment for International Peace.

“To the Iranians, it’s more a negotiation between equals,” he said.

Burns’s presence, along with that of Jake Sullivan, national security adviser to Vice President Biden, is designed to convey U.S. seriousness of purpose. The two participated in secret talks with Iran early last year that led to the current negotiations, and they are seen as trusted by Tehran.

Expectations ran high after an interim agreement was signed in November. Called the Joint Plan of Action, or JPA, it froze Iran’s nuclear expansion — which the West says is designed to build a weapon and Iran says is for peaceful energy purposes — and eased harsh international economic sanctions. The JPA, which was put in force Jan. 20, set a six-month timeline for a permanent, comprehensive agreement.

Although the interim accord envisions an extension of the July 20 deadline if good progress is being made, it will be hard for the administration to convince a skeptical Congress without agreement by Iran on at least one of the most substantive issues.

Earlier this year, a Senate majority signed on to new legislation to ready new, harsher economic sanctions for Iran if the deadline is missed. President Obama dissuaded supporters of the measure by promising that he would introduce such legislation himself if it was clear Iran was not negotiating seriously.

Israel, always dubious of the talks, is likely to push for using force to destroy Iran’s nuclear facilities if progress is not made.

Among the thorniest issues is the number of centrifuges Iran is allowed to retain of its 9,000 operating and 10,000 installed reserve centrifuges, how much enriched uranium it is allowed to produce and to what level of enrichment. Many in Congress and in Israel have argued that the Iranians should be allowed no uranium enrichment capability.

The United States and its partners — Britain, France, Russia, China, Germany and the E.U. — are believed to have indicated a willingness to agree to a 4,000-centrifuge ceiling with a cap of 3 to 5 percent enrichment, for research and development purposes.

But they have argued that Iran has no need to produce large quantities of enriched uranium to operate its Russian-supplied power reactor. The Russians are obligated under the reactor contract to provide fuel until 2021.

Iran, however, feels it has a moral and domestic political imperative to have more. “They say, our people will never accept dismantling things and curtailing [enrichment], because we’ve paid such a price through all the sanctions,” one nuclear expert said, and through the still-unsolved assassinations of Iranian nuclear scientists — killings that are generally blamed on Israel.

A second item of contention is the Arak heavy-water reactor, capable of producing plutonium that can be used to make nuclear weapons, as can a sufficient quantity of very highly enriched uranium. The U.S. position is that the reactor must close, or at the very least be redesigned to produce substantially less plutonium, which the Iranians have argued they need for medical and research purposes.

The timing and phases of restrictions are other issues, as are the insistence by the United States and others that inspections must be extended to military as well as civilian sites.

“It’s a real problem,” the nuclear expert said of the inspections issue. “How do you reassure them you would only call for inspections” when there were real indications of prohibited activity, “and you wouldn’t abuse or use it for intelligence purposes, when in fact we do all those things? And they know it.”

In his video, Zarif blamed the West for Iran’s expanding nuclear program, saying that “crippling” sanctions and “the murder of our nuclear scientists” had compelled them to increase their production of enriched uranium.

“As we approach July 20th,” he said, “I feel compelled to warn again that pursuing a game of chicken in an attempt to extract last-minute concessions cannot achieve anything better” than when a previous round of talks fell apart in 2005.

“Try mutual respect,” he said. “It works.”