The head of the U.N. nuclear watchdog and Iranian officials held a day of high-level talks Monday that both sides described as “positive,” but it remained unclear whether Iran had given ground on allowing access to key nuclear scientists and research facilities.

Yukiya Amano, the head of the International Atomic Energy Agency, traveled to Iran over the weekend for the unusual session and afterward praised his hosts, saying they engaged constructively with the agency. The IAEA has been pressing Iran for years to come clean about alleged nuclear-weapons research by its scientists.

“We had expansive and intensive talks in a positive atmosphere,” Amano told Iranian television after the hastily arranged meeting in Tehran.

Iran’s chief nuclear negotiator, Saeed Jalili, also hailed what he described as “very good talks” with the IAEA leader, adding that, “God willing, we will have good cooperation in the future.”

But neither man spoke in detail about the issue at the heart of the visit: Iran’s refusal to satisfy the nuclear watchdog’s questions about its past nuclear research. Amano hinted at lingering differences, telling reporters that the two sides still have different views about how the controversy should be settled.

“I will not get into details,” Amano said. “The agency has its own view, and Iran has its own.”

Amano’s unexpected trip to Tehran followed a reportedly fruitful round of talks last week between Iran and the IAEA in Vienna. That followed years of complaints about Iranian stonewalling of investigations of its past nuclear activities.

The IAEA has repeatedly been turned back in its efforts to meet with top Iranian nuclear scientists or visit facilities allegedly linked to past nuclear weapons research. Iran denies that it has ever sought nuclear weapons.

Amano’s visit came as Iranian negotiators prepared to travel to Baghdad for closely watched nuclear talks with the P5-plus-1 bloc, made up of the United States, Britain, China, France, Germany and Russia.

At that meeting, scheduled for Wednesday, the six countries are expected to press Iran for deep cuts in its nuclear program, including halting uranium enrichment and shipping much of its uranium out of the country.

Also Monday, the U.S. Senate approved a measure that aims to further squeeze Iran over its nuclear program. The bill increases penalties for energy and uranium mining projects with Iranian firms, and would freeze the assets of businesses that sell Iran technologies that could be used against dissidents, such as tear gas and surveillance equipment.