The report, drawn up by the head of the International Atomic Energy Agency, says Iran has been cooperating with the agency since September, the diplomats said on condition of anonymity.
But disclosures made recently by Iran "clearly show that in the past, Iran had concealed many aspects of its nuclear activities, which resulted in breaches of its obligations of the safeguard agreement" it had signed with the agency, the report says, according to the diplomats.
The report comes as Iran, after months of pressure, declared Monday that it has temporarily halted enrichment of uranium and committed to unfettered inspections of its nuclear facilities, two top demands of the IAEA.
The report by IAEA chief Mohamed ElBaradei will be central when the United States and other member nations on the agency's board of governors meet, starting Nov. 20, to decide what steps to take over Iran's nuclear program.
Washington, which has long accused Iran of seeking to develop nuclear weapons, wants the board to declare Tehran in violation of the Nuclear Nonproliferation Treaty. If the board does so, it will likely kick the issue to the U.N. Security Council, which could impose sanctions.
U.S. officials will likely point to the report's comments on Iranian efforts to hide parts of its program to boost their case.
ElBaradei's report said that so far, the agency had no proof that the hidden programs were involved in weapons production.
"To date there is no evidence that the previous undeclared nuclear material and activities ... were related to a nuclear weapons program," said the report, as cited by one of the diplomats. "However, given Iran's past pattern of concealment, it will take some time before the agency is able to conclude that Iran's nuclear program is exclusively for peaceful purposes."
Iran, which insists its nuclear program aims only to produce energy, gave the IAEA what it called a full accounting of its nuclear activities. It has also been allowing IAEA inspections of some facilities in past months.