Hopes began to fade Wednesday that a fresh round of talks with Iran would help ease tensions over Tehran’s disputed nuclear program after Iran slammed a new package of proposals by Western powers as inadequate.

The package contained what U.S. officials said were confidence-building measures that Iran would need to take to show that its nuclear program is not aimed at producing a weapon, including a reduction in the degree to which the country is enriching uranium, from 20 percent to 5 percent.

But there was no offer of immediate relief from the biting economic sanctions that are hurting Iran’s economy and, notably, no proposal to reconsider a potentially crippling prohibition on Iranian oil exports by the European Union that is to go into effect July 1, a top priority for Tehran.

Iran’s official Islamic Republic News Agency condemned the package as “outdated, not comprehensive and unbalanced.”

“There is no balance, and there is nothing to get in return,” the news agency said.

Talks continued until nearly midnight at a guesthouse in Baghdad’s fortified Green Zone between chief Iranian negotiator Saeed Jalili and representatives of six world powers: the United States, Russia, China, Britain, France and Germany. Plans were made to extend the negotiations into a second day in an effort to find ways to help keep alive this latest diplomatic effort to resolve the concerns about Iran’s nuclear ambitions.

U.S. officials said they are still hopeful that enough common ground would be found Thursday to schedule another round of talks soon. With Israel threatening to strike Iranian nuclear facilities to prevent the Islamic republic from developing the capacity to build a nuclear bomb, many military and security experts have portrayed these latest talks, which began in Istanbul last month after a 15-month hiatus, as a last chance to avert war.

“It has been a difficult day, but I take that as a good sign,” said a senior U.S. administration official, speaking on the condition of anonymity because of the sensitivity of the subject. “It means we have engaged with each other and discussed difficult issues.”

The proposals presented to Iran were intended to ease Western concerns about the country’s nuclear ambitions while offering Tehran a path toward eventual relief from Western sanctions. The six world powers, known as the P5-plus-1, are pressing Iran to immediately give up some of the most weapons-sensitive parts of its nuclear program, including halting its production of a more purified type of enriched uranium that can be easily converted into weapons-grade fuel. Iran also is being asked to ship abroad its stockpile of this 20 percent enriched uranium and eventually shut down a new enrichment plant built into a mountainside near the city of Qom. Much of Iran’s 20 percent enriched uranium is being made there, inside bunkers beyond the reach of most conventional airstrikes.

If Iran agreed, it would receive modest relief from some technology restrictions, such as on imports of aircraft parts, Western diplomats said. Broader relief from sanctions and oil embargoes would come later as part of a more comprehensive agreement on permanent limits to Iran’s nuclear program, the officials said.

Iran countered the proposal with a five-point package, which included broadening the focus of the talks to incorporate the escalating conflict in Syria, which is emerging as a battleground for influence between the United States and its regional allies and Tehran, which is closely aligned to the regime in Damascus.

U.S. officials said they rejected the inclusion of any issue other than Iran’s nuclear program in this round of talks.

Hopes had been raised that the negotiations might produce a breakthrough after the United Nations’ nuclear watchdog announced Tuesday that it was close to a deal with Iran that would open up some of its most secretive nuclear facilities to inspection.

But U.S. officials stressed that the tentative accord reached with the International Atomic Energy Agency pertained to the processes by which Iran might account for the nuclear research programs it has conducted in the past and would not address its plans into the future.

The apparent deal with the IAEA attempts to resolve one of the thorniest disputes between Iran and Western governments in recent years: the nation’s refusal to account for a secret program of alleged nuclear weapons research conducted as recently as 2003. Iran insists that it has never sought to manufacture nuclear weapons, but it has routinely blocked access to key scientists and to military installations where the work was alleged to have occurred.

After a previously unscheduled visit to Iran over the weekend, IAEA Director General Yukiya Amano said Tuesday that the two sides had essentially settled their differences and were formalizing a plan that would ease the investigation of Iran’s past nuclear activities, ending a six-year stalemate.

“I can say it will be signed quite soon,” Amano told reporters at the Vienna airport upon his return from Tehran. Although a few obstacles remain, a “decision was made to conclude and sign the agreement,” he said.

In Washington, the Obama administration cautiously welcomed Amano’s announcement, but several officials noted that similar agreements had fallen apart when Iranian officials refused to provide the promised access.

“It’s an agreement in principle that represents a step in the right direction,” White House spokesman Jay Carney told reporters. “We will make judgments about Iran’s behavior based on actions.”

The Israeli government, which has threatened military strikes against Iran to stop what it sees as Tehran’s pursuit of nuclear weapons, was openly skeptical of the claim of a diplomatic breakthrough.

“The Iranians are trying to reach a ‘technical agreement,’ which will create the impression of progress in the talks,” Israeli Defense Minister Ehud Barak said at the start of a Ministry of Defense meeting. By appearing to make concessions, Iran is seeking merely to deflect international pressure on itself, he said.

Neither Iran nor the IAEA provided details of the accord, although Amano spoke of progress on a “structural agreement” that laid out the terms under which Iran would give the agency information about its past nuclear research.

Jalili, the chief Iranian negotiator, spoke vaguely about what he said were “very good talks” with the U.N. nuclear agency. “God willing, we will have good cooperation in the future,” he added.

To some former U.S. officials and arms-control experts, the apparent progress at the Tehran meeting was a positive sign.

“Now the task is to reach agreement on specific, concrete proposals, followed by actions, that can help prevent a nuclear-armed Iran,” said Daryl Kimball, executive director of the Arms Control Association, a Washington-based nonprofit organization.

Current and former Obama administration officials acknowledged that the chances for a comprehensive agreement Wednesday were slim, given the complexity of the issues and the time needed for consultations between the negotiators and their governments. But several officials said they expected at least to have firm indications from Iran about its willingness to address Western concerns.

“One doesn’t need to see a breakthrough in these talks — it’s not realistic,” said Dennis Ross, who until last fall was President Obama’s chief adviser on Iran. “But you need to see indicators that they are willing to talk about some of these things.”

Warrick reported from Washington.