Iran suspended its most sensitive nuclear development work Monday and world powers immediately responded by lifting some of the sanctions that have crippled the oil-based economy — a first cautious step toward making good on a deal aimed at resolving a decade-long standoff over Iran’s nuclear program.

The reciprocal steps honor a plan to gradually ease restrictions on Iran as it adheres to new commitments to curb its program and allow greatly expanded international inspection and monitoring of the program that it insists is not intended to produce nuclear weapons.

The United States, a chief skeptic of Iran’s claims and long the main international driver for tough sanctions on Iran, reacted with both praise and skepticism, saying it was an “important step forward” but also warning that the restrictions would be quickly reinstated if Iran reneges on its efforts.

The United Nations’ nuclear watchdog agency on Monday said Tehran has stopped enriching uranium beyond a 5 percent potency level and disabled connections between centrifuges that had been producing 20 percent-enriched uranium. Iran has also begun diluting half its stockpile of that higher-level uranium, which is considered within striking distance of bomb-quality fuel.

The announcement in Tehran met a Jan. 20 deadline for the move, which is intended to lay groundwork for a broader accord on Iran’s nuclear activities.

The European Union said it was responding with a release of some sanctions as part of “a six-month phase of initial confidence-building measures aimed at addressing international concerns about Iran’s nuclear activities.”

The decision, which was made by European foreign ministers at a meeting in Brussels, was expected to allow Iran to resume critical exports of oil and gas.

“This is an important day in our pursuit of ensuring that Iran has an exclusively peaceful nuclear program,” E.U. foreign policy chief Catherine Ashton told reporters.

The White House issued a more reserved statement.

“In reciprocation for Iran’s concrete actions,” White House press secretary Jay Carney said in a statement, world powers would “today follow through on our commitment to begin to provide the modest relief agreed to with Iran. At the same time, we will continue our aggressive enforcement of the sanctions measures that will remain in place throughout this six-month period.”

Through its skepticism, the Obama administration is trying to address a chorus of critics at home and in Israel who say the deal is too generous as it combats efforts in Congress to create new sanctions, while the United States and other Western powers try to reach a long-term deal with Iran.

Iran would receive about $4.2 billion in sanctions relief over the six-month period to reach a final deal, plus perhaps $2 billion in associated trade, U.S. officials said. Senior Obama administration officials have called the relief a “drop in the bucket” compared with the billions of oil revenue that the global sanctions on Iran have left tied up in international banks.

“Iran is not truly open for business,” one U.S. official said, noting that remaining oil sanctions will cost Iran about $30 billion over the same period.

Under the sanctions, most of Iran’s legitimate oil trade was suspended over the past two years. European and other corporations are widely reported to be seeking ways to resume business with Iran soon, leaving U.S. officials trying to stand in the breach.

U.S. officials, who spoke on the condition of anonymity to describe the technical agreement and the closed-door negotiations with Iran, also addressed the fraught politics of the Iran deal in the United States, where a bipartisan and nearly veto-proof majority of senators is on record supporting new sanctions on Iran. The administration opposes new sanctions as a violation of its careful agreement with Iran and other world powers and warns that Iran may walk away if Congress won’t back down.

“The steps that we are taking today shows that we are on track toward implementing this arrangement which gives us what we need, which is halting for the first time in more than a decade Iran’s progress toward nuclear weapons capability,” one official said.

“It just seems all the more clear to us that we should test the proposition that moving forward with this will give us in the end what we need, which is a comprehensive solution” that assures Iran cannot build a weapon, the official said.

The nuclear talks are separate from other U.S. and world political disputes with Iran, including whether Iran participates in upcoming peace talks on the Syrian civil war, the official said.

The United Nations on Monday rescinded an invitation to Iran to attend the talks because Iran would not endorse a transition from the regime of Syrian President Bashar al-Assad, an ally of Tehran.

In Iran, there was mixed reaction to the halting of the uranium enrichment at nuclear plants in the cities of Natanz and Fordo.

Iranian opponents of the deal stepped up their denunciations of what they are calling their country’s capitulation to Western demands.

Vatan-e Emrooz, a newspaper closely associated with conservatives, printed Monday’s edition in all-black type and dedicated it to coverage of what it called Iran’s “nuclear holocaust.”

“What we have given up is not only incomparable with what we have received, but much less significant than can be called a win-win situation,” wrote Hossein Shariatmadari, the editor in chief of Kayhan, a newspaper often referred to as the mouthpiece for the most conservative members of Iran’s political establishment.

Criticism of the deal was not unexpected, but opponents of Iran’s new president, Hassan Rouhani, and his administration’s nuclear outreach had been uncharacteristically quiet until now.

Ahmad Tavakoli, a prominent conservative member of parliament, told fellow lawmakers Monday that there were two main problems with the nuclear deal.

“First of all, there are so many discrepancies in the text of the agreement that we can hardly be hopeful that our national interests will ever materialize. Second, as officials, we must not reveal our weak points in a way that our enemies can exploit them,” Tavakoli said.

Despite the intensified conservative backlash, the agreement is supported by many key Iranian political figures, as well as many ordinary Iranians who hope that an easing of sanctions will ease economic woes that have reached deep into the Iranian middle class.

Early indications show that Iran’s financial markets are responding well to the deal’s implementation as the Iranian currency, the rial, strengthened by more than 2 percent against the dollar Monday.

Gearan reported from Washington. Griff Witte in London contributed to this report.