The head of the United Nations atomic watchdog said it has had limited progress in its inquiry into the military aspects of Iran's nuclear program, in part because Tehran has failed to provide information and access the agency needs to verify its nuclear capacity.

Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said in a recent interview that Iran has only replied to one of its dozen inquiries about the "possible military dimensions" of the country's past nuclear activities, reported The Washington Post. On two other issues, Iran has only provided "very limited" information, according to Amano.

"Recently the progress is very limited," he said.

IAEA's long-standing inquiry into allegations that Iran could have engaged in work developing nuclear bombs runs parallel to talks currently underway between Tehran and the P5+1 nations.

World powers are currently in the final stages of negotiating an agreement with Iran, which would stop some aspects of the country's nuclear program in exchange for the U.N. providing sanction relief. An interim deal was reached in 2013, and the IAEA was tasked with making sure that Iran is abiding by the terms of that agreement, including not using civilian energy technology to produce nuclear weapons, according to Reuters.

"As far as the implementation of the comprehensive safeguards agreement, we have some good cooperation with Iran," Amano said.

But as the Post noted, if Iran refuses to comply with IAEA requests for access or information, it risks jeopardizing the lifting of U.N. sanctions.

Amano stressed that IAEA has not accused Iran of having nuclear weapons, but said he believes that the P5+1 nations should insist that Iran allow IAEA inspectors to visit any of its facilities at any time to examine for evidence of secret nuclear weapons development.

"With respect to the clarification of issues with possible military dimensions, the progress is limited and this is the area where more ... cooperations from Iran is needed," Amano said.

Part of IAEA's investigation involves determining whether Iran has diverted nuclear material away from peaceful operations, like power generation, and directed it towards military purposes, like making a nuclear bomb. Amano said IAEA hasn't been able to verify whether Iran has done this, Reuters reported.