The IAEA says Iran is operating 3,500 centrifuges at its plant at Natanz
The UN nuclear watchdog has said it believes Iran is still withholding information on its nuclear programme.
In a report, the International Atomic Energy Agency (IAEA) says Tehran's alleged weapons development studies remain a matter of serious concern.
It adds that Iran is operating 3,500 centrifuges, the machines used to enrich uranium, at its plant at Natanz.
Enriched uranium can be used in nuclear weapons, but Tehran insists its nuclear programme is entirely peaceful.
Last month, the IAEA said it had reached an agreement with Iranian officials to clarify the main outstanding question about Iran's past nuclear work by the end of May.
But in its latest report, the IAEA says Tehran needs to provide much more explanation and information on its nuclear activities.
Responding to the report, Iran's envoy to the IAEA told the AFP news agency his country had "left no question unanswered" and would continue to enrich uranium.
"Iran has not provided the agency with all the information, access to documents and access to individuals necessary to support Iran's statements [that its activities are purely peaceful in intent]," the IAEA report said.
A vindication and reiteration of the peaceful nature of Iran's nuclear activities
Iran's description of the new IAEA report
"The agency is of the view that Iran may have additional information, in particular on high explosives testing and missile-related activities which... Iran should share with the agency.
An unnamed "senior UN official" in Vienna told Reuters news agency:
"We have not got substantive answers and we could have gotten those earlier. It's up to Iran [now]."
Gregory Schulte, the chief US delegate to the IAEA, said the report had detailed a "long list of questions that Iran has failed to answer".
"At the same time that Iran is stonewalling its inspectors, it's moving forward in developing its enrichment capability in violation of Security Council resolutions," he told The Associated Press.
The envoy described parts of the report as a "direct rebuttal" of Iranian arguments that all nuclear questions had been answered.
Ali Asghar Soltanieh, Iran's envoy, told AFP by telephone that "200 pages of explanations" had been furnished by Tehran.
The IAEA report was, he said, "a vindication and reiteration of the peaceful nature of Iran's nuclear activities".
"We will continue enrichment, while not suspending our cooperation with the IAEA," he added.
Iran has told the IAEA it plans to have 6,000 uranium-enriching centrifuges operating by the end of the summer.
Footage of Iranian nuclear facilities
Speaking anonymously, officials close to the IAEA said on Monday they had no reason to doubt this was the case.
The suspension of uranium enrichment is a key demand of the UN Security Council, which has imposed sanctions on Iran.
Uranium can be used either as nuclear fuel or as the fissile core of missile warheads, depending on the degree of enrichment.
Iran insists it has a right to enrich uranium under the Nuclear Non-proliferation Treaty and says it want only to generate power.
Iran's nuclear programme has been under IAEA investigation since 2002, when Iranian dissidents revealed the existence of secret uranium enrichment.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.