A UN report says Iran now has enough uranium to make an atomic bomb.
The UN's nuclear watchdog, the IAEA, says Iran has amassed more than 1,300kg of uranium - well over the amount needed to make a single atomic bomb.
Iran has ignored UN resolutions ordering it to stop enriching uranium, saying its programme is peaceful.
In a separate report about Syria, the IAEA says particles of manmade uranium have been found at a research reactor in the capital, Damascus.
In a confidential report obtained by the BBC, it says the uranium is of a type not included in Syria's declared nuclear material.
The agency is looking into US intelligence reports that Syria had almost built a North Korean-designed nuclear reactor capable of yielding weapons-grade plutonium before Israel bombed it in 2007.
Last year the IAEA found particles of manmade uranium at the remote al-Kibar site in northern Syria.
The UN Security Council wants Iran to stop all enrichment programmes pending an IAEA investigation into its uranium development programme.
In its report, the agency said Iran was still defying the Security Council and had so far amassed 1,339kg of low-enriched uranium at its plant in Natanz.
Analysts calculate that anywhere between 1,000-1,700kg of low-enriched uranium is enough to make one atomic bomb.
There were still "outstanding issues which give rise to concerns, and which need to be clarified to exclude the existence of possible military dimensions to Iran's nuclear programme", the IAEA said.
The agency says Iran is refusing to allow more rigorous inspections and was blocking requests for access to "relevant information".
Israel has expressed alarm over Iran's nuclear programme, particularly after President Mahmoud Ahmadinejad called for the Jewish state to be "wiped from the map".
However, earlier this week Foreign Minister Avigdor Lieberman said Israel did not intend to bomb Iran, adding: "It is not a problem for Israel, it is a problem for the Middle East."
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