The report from the International Atomic Energy Agency also reiterated that more cooperation is needed by Iran for full clarity on its present activities. Without it, the IAEA said it cannot “conclude that all nuclear material in Iran is in peaceful activities.”

After years of deadlock, Iran and the IAEA agreed in November 2013 on a new attempt to probe the accusations. The U.S. and its allies also included the investigation into a to-do list for talks with Iran meant to curb its nuclear programs in exchange for sanctions relief.

Washington continues to insist that full lifting of sanctions depends on the IAEA’s ability to thoroughly probe the accusations and deliver an assessment on its findings.

Iran denies any work on – or interest – in nuclear arms. It accuses Israel, the United States and other adversaries of providing phony evidence to the agency for the probe.

The IAEA is focused on 12 alleged activities that point to Iranian attempts to make such weapons, including suspicions that Tehran worked on the development on a nuclear payload for missiles. The IAEA relaunched its probe two years ago by asking for information on less sensitive work related to nuclear arms allegedly carried out by Iran, with hopes of moving to larger issues later.

U.S. Secretary of State John Kerry will meet with Iranian Foreign Minister Mohammad Javad Zarif in Geneva on Saturday in an effort to move the nuclear talks forward ahead of a June 30 target date for a deal. The confidential report issued Friday by the IAEA increased doubts that the agency could issue substantive findings on the allegations by that time.

Obtained by The Associated Press, the report said that Tehran had recently shared some information sought by the agency but continues to hold back on the next stage of the IAEA probe.

A senior diplomat with knowledge of the issue said the material was related to nuclear modeling and calculations that the agency suspects could be linked to arms. The diplomat, who demanded anonymity because his information was confidential, declined to say whether the information was presented by Iran as proof that the suspicions were false or if they possibly pointed to corroboration of secret nuclear weapons work.

In providing information on another issue last year – explosives-related experiments – Iran insisted the material proved the testing was for civilian uses and were not related to the alleged work on nuclear weapons. The agency has not issued a public opinion on that assertion.

Since August, “Iran has yet to propose any new practical measures” to bring the investigation forward, said the report.

The agency seeks to interview officials suspected of involvement in weapons research and development. It also wants to visit sites allegedly used for such work. But as the Iran-six power nuclear talks approach an end of June target date, Iranian officials are adamant that they won’t allow such questioning or inspections. Iranian Deputy Foreign Minister Abbas Araghchi called his country’s rejection of access to such individuals and locations “totally nonnegotiable” on Friday.

The IAEA report, issued to the agency’s 35-nation board and the U.N. Security Council, said it remains “concerned about the possible existence in Iran of undisclosed nuclear-related activities involving military-related organizations, including activities related to the development of a nuclear payload for missiles.”

“Iran is required to cooperate fully with the Agency on all outstanding issues.”