LAUSANNE, Switzerland—Iran and world powers on Thursday announced that following the latest round of nuclear negotiations, Western powers agreed to permit Iran to continue operating the core aspects of its nuclear program and that all sanctions of the Islamic Republic would be terminated.

After failing to meet a March 31 deadline for the announcement of a firm political agreement, Secretary of State John Kerry and Javad Zarif, his Iranian counterpart, said that the sides had agreed in principle to let Iran continue running major portions of its nuclear program.

Despite threats from Obama administration officials that the United States would abandon talks if Iran continued to demand greater concessions, Kerry extended his trip and conducted a series of meetings aimed at hashing out a statement of progress—a far cry from the detailed document officials vowed would be finalized by now.

The sides continue to disagree over Iranian demands that it be permitted to continue key nuclear research and granted the ability to ramp its program up to industrial capacity after a decade.

However, Zarif said many of these issues are closer to being resolved.

"None of those measures" that will move to scale back Iran’s program "include closing any of our facilities," Zarif said. "We will continue enriching; we will continue research and development."

"Our heavy water reactor will be modernized and we will continue the Fordow facility," Zarif said. "We will have centrifuges installed in Fordow, but not enriching."

The move to allow Iran to keep centrifuges at Fordow, a controversial onetime military site, has elicited concern that Tehran could ramp up its nuclear work with ease.

Zarif said that once a final agreement is made, "all U.S. nuclear related secondary sanctions will be terminated," he said. "This, I think, would be a major step forward."

Zarif also revealed that Iran will be allowed to sell "enriched uranium" in the international market place and will be "hopefully making some money" from it.

European Union High Representative Federica Mogherini said in a statement that the sides had "taken a decisive step" in paving over disagreements on key fronts and would now work "for a final deal" by June.

In return, the United States and international partners will work to "terminate" all of the "nuclear-related sanctions" currently being imposed on Iran.

The United Nations also will move to endorse the ongoing Joint Plan of Action interim deal and terminate all of its previous security council resolutions on Iran.

The Fordow nuclear facility will eventually be "converted from an enrichment site to a nuclear psychics and tech center," Mongherini said. No fissile material such as uranium will be permitted at the former military site.

However, this could allow Iran to continue running thousands of nuclear centrifuges that could be used to enrich uranium to weapons-grade levels.

"As Iran pursues a peaceful nuclear program," it will tentatively agree to limit its nuclear enrichment capabilities and stockpiles for a specific duration, according to Mongherini’s statement, which was also read in Persian by Zarif.

Iran also will undertake a "joint venture" to redesign and modernize its heavy water reactor at Arak, a site that has raised concerns about the plutonium path to a nuclear bomb.

The site "will not produce weapons-grade plutonium," according to the statement.

"We’ve done significant work," Zarif said after the joint statements had been read. "We’ve taken a major step, but we’re still some time away from reaching where we want to be."

Many concessions Iran has pushed for in recent days have now been agreed to by the United States, according to multiple reports and sources.

Prior to the news conference, Zarif informed the Iranian state-controlled press that "no agreement will be signed" in the near term.

"We have said right from the beginning that no agreement would be signed today. We have always stated that there could be only one agreement which could go into effect at the end of the talks on July 1 if everything goes well," he was quoted as saying.

Zarif told reporters late Wednesday amid meetings with the United States and other P5+1 one nations that he was "all smiles" after days of intensive talks in which Tehran has given little ground on American efforts to reduce the size of its nuclear program and uranium stockpiles.

Zarif said no progress could be made in the talks due to continued American "pressure" and lack of respect.

Iran went back on earlier promises that it would export its stockpiles of enriched uranium, the key component in a nuclear weapon. This requirement remains one of the issue's sticking points.

Secretary of State John Kerry told reporters following Zarif’s remarks that the United States has "reached a crucial milestone" with Iran.

Kerry celebrated what he described as major agreements by Iran to limit the scope of its enrichment ability.

"We will give the international community confidence that’s Iran’s nuke program is and will remain exclusively peaceful," Kerry said. "I can tell you the political understanding with details we have reached is a solid foundation for the good deal we are seeking."

Kerry also said that Iran is not fully complying with a portion of the agreement, economic sanctions would be reimposed.

However, differences still remain, he said.

"We have acknowledged there are some gaps," Kerry told reporters. "There are issues we have to resolve."

The differences will be worked over the coming months, he said.

With the talks slated to continue through June, a majority of Americans said Congress "should be required" to approve the agreement, something the Obama administration has opposed.

Around 55 percent of those surveyed by Fox News said the United States cannot "trust anything" Iran promises on the nuclear front.