Senior officials of the UN atomic agency have returned from Tehran without a hoped-for deal that would have led to the resumption of a probe into allegations that Iran worked secretly on nuclear arms.

Herman Naeckerts, who headed the International Atomic Energy Agency team, says “remaining differences” between the two sides mean that “we… could not finalize” the agreement on how such an investigation should be conducted. He declined to say whether there was progress.

Iran denies it ever worked on nuclear arms and says all of its atomic activities are peaceful. The talks began more than a year ago, but have foundered on modalities.

The IAEA wants the probe to be open-ended, something Tehran strenuously opposes.

Senior UN investigators were in Iran Wednesday for a new round of talks with government officials over allegations that Tehran may have carried out tests on triggers for atomic weapons.

The visit by the UN team comes a day after Tehran raised prospects that the IAEA may be allowed to inspect Parchin — a military site where the agency suspects nuclear-related experiments were conducted — if certain conditions were met.

However, IAEA director general Yukiya Amano said on Monday that “the outlook is not bright” for a Parchin visit, according to The New York Times. Iran denies any testing activity at Parchin and insists the facility is a conventional military site only.

The possibility of Iran nuclear tests came under greater scrutiny this week, after North Korea successfully tested a nuclear device on Tuesday, setting off a wave of international condemnation. North Korea and Iran are suspected to have shared nuclear technology.