In what Obama administration officials called the most significant progress in years, Iran and six world powers concluded the first round of talks on Iran's nuclear program since Iranian president Hassan Rouhani ascended to power in August.

The talks, which ended Wednesday night in Geneva, were "productive" and useful, despite the lack of any major breakthroughs, U.S. and European said.

"I've been doing this now for about two years, and I have never had such intense, detailed, straightforward, candid conversations with the Iranian delegation before," a senior Obama administration official told reporters in Geneva.

The discussions took place in English, which had never occurred before, the official noted.

Senior U.S. and Iranian officials also met on their own, without their European counterparts, something that hasn't happened in four years. All parties agreed to meet again in three weeks.

Secretary of State John Kerry and Iranian Foreign Minister Javad Zarif had met last month at the United Nations and announced that the negotiations on Iran's nuclear program would restart.

The Iranians are seeking a deal that allows them to continue to enrich uranium for energy production and peaceful means, at not more than 20 percent, and an end to the crippling sanctions that have put the country on the brink of financial ruin. President Obama has repeatedly said a nuclear-armed Iran would be unacceptable to the United States and to the world.

The United States wants international inspectors to have unfettered access to Iran's nuclear sites and more transparency from the regime about how far its enrichment program is progressing. Iran, particularly under Rouhani's predecessor, Mahmoud Ahmedinejad, was consistently accused of misleading or deceiving the international community about the nature of its nuclear program.

Obama has also said the United States will continue taking a "dual track" policy with Iran, which entails maintaining sanctions, even while negotiations occur, until Iran can prove it is not seeking to build a nuclear weapon. For its part, Iran has not offered to halt its program while negotiations are ongoing.

"The key challenge facing diplomacy is how to sequence concessions," says Cliff Kupchan, the director of Eurasia's Middle East program. "Iran wants sanctions relief up front, while the U.S. insists on significant constraints on Iran's program from the outset. The sides still need to square that circle."

Still, that all parties have agreed to meet again is a sign that there was enough progress to continue to keep the talks going. Iranian officials have stated publicly that they would like a deal within the year.

U.S. officials wouldn't characterize whether such a timeline is realistic but did say that the Iranians seem more serious about a real proposal than in years past.

"There is more work, much more work to do," the senior official said. "This is a beginning. Beginnings are rarely groundbreaking because you are putting pieces on the table."

But analyst Kupchan says that the next meeting will need to have a tangible breakthrough to hold off members of the Senate who want to impose more sanctions on Iran, and hardliners who might grow restless with waiting for the international community to ease sanctions.

"Given the long history of failed Iran-U.S. diplomacy," he said, "neither side's elites will wait long for results."