(CNN) -- Iran is not cooperating, making it difficult for the UN's nuclear watchdog agency to provide "credible assurance" that the country doesn't possess undeclared nuclear material, the International Atomic Energy Agency Director General Yukiya Amano said Monday.
Speaking to the IAEA's board of governors, Amano said Iran should grant access "without further delay" to the Parchin military complex, where the Islamic Republic is believed to have tested rockets.
Since Iran has not provided such access to date, "The Agency therefore cannot conclude that all nuclear material in Iran is in peaceful activities," Amano said.
For about a decade, international observers have accused Iran of secretly pursuing a nuclear weapons program, which Iran consistently denies.
Amano noted that Iran has recently begun installing IR-2m centrifuges at its fuel enrichment plant at Natanz, and that those high-speed devices that rotate to enrich uranium are more advanced than the previously-installed centrifuges.
The IAEA board has been trying to negotiate cooperation from Iran for years.
On Wednesday, diplomats from the so-called P5+1 -- the six-nation diplomatic bloc consisting of the United States, France, Britain, Germany, China and Russia -- held secretive talks with Iranian representatives in Kazakhstan, and then announced future meetings on March 18 and April 5.
The Islamic Republic already faces sanctions from the United Nations Security Council for violating a U.N. resolution forbidding it from enriching uranium.
But Iran says since it signed the Nuclear Non-Proliferation Treaty, it should be able to enrich uranium for peaceful energy needs.
The IAEA was created in 1957 to promote "safe, secure and peaceful nuclear technologies," its website says. Among its endeavors, the agency tries to verify that countries correctly and completely account for their nuclear material and activities.
The agency says as of December 31, it had "safeguards agreements in force" with 179 official countries, and Taiwan.
CNN's Jennifer Deaton and Ivan Watson contributed to this report