U.N. inspectors have agreed to return to Tehran later this month for a second round of talks on Iran’s nuclear program, while acknowledging that meetings this week achieved no significant breakthroughs, diplomats said Wednesday.

A team of technical experts for the International Atomic Energy Agency announced plans for a second visit on Feb. 21 after wrapping up three days of talks in the Iranian capital. The agency is using the unusual meetings to press Iran to account for controversial nuclear research it conducted in the past.

Diplomats briefed on this week’s visit described the meetings as cordial but said Iran had not yet provided the team with explanations for past scientific projects that appear related to nuclear warhead design. Iran has denied ever seeking to build nuclear weapons.

“This was just a start,” said one European diplomat, who insisted on anonymity in keeping with diplomatic protocol.

Speaking with reporters after arriving at the Vienna airport, delegation leader Herman Nackaerts said the U.N. team was “committed to resolve all the outstanding issues,” the Associated Press reported. “And the Iranians said they are committed, too.”

U.S. officials have welcomed the new talks, though some have expressed concern that the Iranians might stall for time by scheduling meetings that give a pretense of cooperation, while Iran continues to build its stockpile of enriched uranium. Nuclear weapons experts fear that Iran already has acquired most of the technology it needs to build a nuclear weapon if it decides to do so.

IAEA Director General Yukiya Amano has repeatedly pushed the Iranians to come clean about their past research, which is described in hundreds of pages of secret documents provided to U.N. officials by intelligence agencies, including those of the United States. Some of the documents appear to show Iran experimenting with sophisticated detonators of the type used to trigger nuclear explosions.

Amano, in statement Wednesday, said the agency was “committed to intensifying dialogue” with Iranian officials over the studies.

“It remains essential to make progress on substantive issues,” he said.

The IAEA visit did not include trips to any of Iran’s nuclear facilities, which are under regular inspection as well as video monitoring by other IAEA teams.

In Tehran, Foreign Minister Ali Akbar Salehi said he had been prepared to allow the IAEA visitors to tour nuclear facilities, “but they did not want to do so,” Iran’s official Islamic Republic News Agency reported.

Salehi also reiterated that his government was prepared to resume international talks on Iran’s nuclear program with the so-called P-5 Plus One — the five permanent members of the U.N. Security Council plus Germany. Iranian leaders have previously expressed a willingness to hold negotiations, but no formal request for talks has been relayed by Iranian negotiators.

The renewed diplomatic activity comes a day after the release of a U.S. intelligence report that concludes that Iran may attempt to launch terrorist attacks inside the United States in response to continued international pressure over its nuclear program.

Director of National Intelligence James R. Clapper Jr. told a congressional panel that Iran appears to have crossed a threshold in its adversarial relationship with the United States, and may be emboldened to conduct attacks in response to “real or perceived U.S. actions that threaten the regime.”

There is no intelligence indicating that Iran is actively plotting such attacks, Clapper said.