VIENNA (AP) — The head of the U.N. nuclear watchdog agency said Monday he cannot guarantee that Iran is not trying to develop atomic arms, comments that reflect the lack of progress in his attempts to probe Tehran’s nuclear secrecy.

Yukiya Amano, who heads the International Atomic Energy Agency, told the 35-nation IAEA board that Tehran is refusing to cooperate with an IAEA probe of its programs.

“Iran is not providing the necessary cooperation to enable the agency to provide credible assurance about the absence of undeclared nuclear material and activities in Iran,” Amano told the board’s opening session.

That, he said meant that the IAEA cannot “conclude that all nuclear material in Iran is in peaceful activities.”

His comments were similar to previous statements, pointing to a lack of IAEA progress since he took office 15 months ago to prove or disprove U.S. and other intelligence alleging hidden experimental nuclear weapons work by Iran.

Because Iran also refuses to give the agency more intrusive inspecting powers it also cannot follow up on concerns that Tehran might be enriching uranium at sites it has not declared.

The U.N. Security Council has slapped four sets of sanctions on Tehran for its refusal to stop enriching, which Iran says it needs to make fuel for a future reactor network but which can also be used to make the fissile core of nuclear warheads.

Enrichment for peaceful purposes is allowed under the Nuclear Nonproliferation Treaty. But the Islamic Republic’s nuclear secrecy has added to concerns. Beyond refusal to cooperate with the IAEA program of alleged nuclear weapons programs experiments, its enrichment program was clandestine until a dissident group revealed it nearly a decade ago.

In comments made available to reporters, Amano also told the closed meeting that Syria continues to stonewall an investigation of a site suspected to have been a secretly built reactor that would have produced plutonium once finished. Israeli warplanes destroyed that structure three years ago.