Iran's stockpile of low-enriched uranium gas dropped below the maximum level required under a 2013 interim nuclear agreement with world powers, a U.N. report showed, but a US think-tank suggested Tehran had not entirely met its obligations.

The International Atomic Energy Agency (IAEA) said in its monthly report on Iran, a confidential document seen by Reuters on Wednesday, that Iran's stockpile of uranium gas enriched up to a fissile purity of 5 percent was at 7,537 kg at end-June - below a roughly 7,650 kg ceiling stipulated in the November 2013 interim nuclear deal with six world powers.

A US-based think-tank, however, issued an analysis of the IAEA report that questioned whether Iran had indeed complied with the requirement to convert its low enriched uranium (LEU) to a form with less risk of proliferation, uranium dioxide.

"The IAEA's recent report on the implementation of (the interim deal) shows that only 9 percent of Iran's stockpile of newly produced LEU hexafluoride has actually been converted into uranium dioxide form," the Institute for Science and International Security said in a press release.

"When it became clear that Iran could not meet its commitment to convert the LEU into uranium dioxide, the United States revised its criteria for Iran meeting its obligations," the institute said, adding that the LEU had apparently been converted into a form different from uranium dioxide.

"Iran had two requirements under the (interim deal): to end the time period with the same amount of UF6 they began it with, and to convert any excess UF6 produced into an oxide form. They've done both," a senior US official told Reuters.

The IAEA did not have an immediate response to a query about its report. – Reuters