TEHRAN, Iran — Iran suspended part of its uranium enrichment efforts Monday and the U.S. and European Union began loosening some economic sanctions, the first concrete steps toward a comprehensive deal to end the long confrontation over Tehran’s nuclear ambitions.

But even as a six-month interim accord began on schedule, opposing sides differed sharply on the implications of what had been achieved as they tried to win over skeptics at home.

Iran’s leaders, eager to sell their economically beleaguered public on the benefits of nuclear negotiations, said the agreement would open the way to a surge of international trade and investment.

“The window of opportunity for Iran’s trade with Europe will increase tenfold” once the deal is fully in place, Abbas Araqchi, Iran’s chief nuclear negotiator, told the official Islamic Republic News Agency. “The private sector of Iran will have a great share of trade with the European Union.”

Ali Akbar Salehi, head of Iran’s nuclear agency, struck an even more triumphal tone: “The iceberg of sanctions is melting while our centrifuges are also still working,” he said on state television. “This is our greatest achievement.”

U.S. officials, keen to rebut criticism among some lawmakers and others that the deal negotiated in November gave up too much to the Iranians, insisted that the sanctions relief was small in scale and easily reversible if Iran reneged. They stressed that other crippling restrictions on Iran’s economy remain in place.

“Iran is not open for business,” said a senior official briefing reporters on condition of anonymity because of diplomatic sensitivities.

Unless the two sides reach a final agreement this year designed to prevent Iran from gaining the ability to build nuclear weapons, the full raft of sanctions against Iran’s economy won’t be lifted, the official said.

While negotiations proceed, the Obama administration will continue to enforce sanctions unaffected by the interim agreement “in a very aggressive manner,” the official said. “Nobody should misunderstand that point.”

The six-month agreement, signed Nov. 24 by Iran and six world powers _ the United States, Russia, China, Britain, France and Germany _ was designed to buy time to negotiate a more permanent accord. It calls for Iran to roll back parts of its nuclear program in exchange for limited relief from international sanctions. Though scheduled to last six months, the deal can be renewed for another six as the two sides talk.

Iran says its nuclear efforts are strictly for peaceful purposes, including energy generation and production of medical isotopes for cancer treatment. The United States and other nations fear the real goal is to develop a nuclear weapon, leading to the series of sanctions that have helped saddle Iran with high unemployment, galloping inflation, a collapsing currency and widespread public discontent.

Tehran’s nuclear program is just one of many issues that have raised concern among Western officials. Iran backs Syrian President Bashar Assad as he faces a nearly 3-year-old rebellion supported by the U.S. and its allies, is a key backer of the Hezbollah movement in Lebanon, and rivals Saudi Arabia and other Arab allies of the U.S. for influence in the region.

The carefully choreographed first steps of the interim agreement began Saturday when inspectors from the International Atomic Energy Agency, or IAEA, arrived in Iran to oversee implementation.

On Monday, Iranian officials said they had disconnected centrifuges at two of the nation’s nuclear sites, Natanz and Fordow, fulfilling the requirement to end production of uranium enriched to 20 percent, a medium level of purity. Iran has also started reducing its stockpile of 20 percent enriched uranium, which critics warned could be enriched further to produce weapons-grade material, officials said.

In a statement reported by Press TV, Iran’s official English-language news service, Salehi said the Islamic Republic had started to “voluntarily suspend” medium-level enrichment in compliance with the accord.

“This is an important first step, but more work will be needed to fully address the international community’s concerns,” said Catherine Ashton, the EU’s top diplomat, who was instrumental in brokering the November agreement.

Ashton said the next round of negotiations will start in February. Wendy Sherman, the head of the U.S. negotiating team, is to visit Geneva on Tuesday to begin talks with the five other world powers on how to approach the bargaining, U.S. officials said.

Iranian energy officials have been holding out the lure of big deals to overhaul the country’s aging energy infrastructure and have been hinting that they are willing to structure terms to bring big payoffs to multinational oil companies.

In Iran, the start of the nuclear deal was greeted with predictable pessimism from hard-line commentators, who called on the government to seek revisions in the accord to protect the nuclear program. Many suspect that negotiators gave up too much.

“I thank our diplomats, who love the homeland, but I must say there are many problems in the texts of the agreements,” said parliament member Ahmad Tavakkoli in a speech to lawmakers, Iran’s Fars News Agency reported. “One can hardly hope that the national interests are secured in these conditions.”

But the deal seemed generally well received on the streets of Tehran, where many are looking to improved economic prospects and a thaw with the West.

“I do hope it will lead to stability and predictability in our economy,” said Ali Gholami, a 40-year-old motorcycle courier. “It may take two years to stabilize the economy, but it is better than nothing. I wish we had done this eight years ago.”

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