But others drew hope from the fact that the two sides saw reasons to continue talking.

“An initial confidence-building deal is still within reach if both sides show some flexibility,” said Daryl Kimball, executive director of the Arms Control Association, a Washington nonprofit.” While an agreement on initial confidence-building steps was not reached in Baghdad, it is clear that both sides are exchanging serious proposals that could produce results in the next round.”

Talks continued until nearly midnight Wednesday at a guesthouse in Baghdad’s fortified Green Zone between chief Iranian negotiator Saeed Jalili and representatives of the six world powers: the United States, Russia, China, Britain, France and Germany. The negotiations were originally scheduled to last one day, but the parties ultimately agreed to extend them into Thursday in hopes of keeping the diplomatic effort alive.

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 portrayed the 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,” a senior U.S. administration official said Wednesday, speaking on 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 its production of enriched uranium. Iran also is being asked to ship abroad its existing stockpile of 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.