The United Nations nuclear watchdog says Iran is continuing to obstruct its investigation into allegations of past work on nuclear weapons, but the country's uranium enrichment program is expanding more slowly than expected.

The report by the International Atomic Energy Agency, due to be released today, is likely to sharpen debate within the Obama Administration, which is reviewing its Iran policy.

Previewing the report, the director-general of the IAEA, Mohamed ElBaradei, gave a scathing assessment of Iran's co-operation with its inquiries into the country's past nuclear experimentation, alleged by the United States to be aimed at building weapons. "Iran, right now, is not providing any access or any clarification with ... those studies or the whole possible military dimension," he said in Paris.

The report will also make clear that Iran is still defying UN Security Council demands to stop enriching uranium, despite three sets of UN sanctions. But Mr ElBaradei said Iran had not significantly expanded its enrichment program at its Natanz plant in the past few months, in what he suggested might be a deliberate attempt not to stoke tensions too far.

Western diplomats urged caution over Mr ElBaradei's conclusions, saying Iran's enrichment program has varied in speed over the years but has relentlessly expanded in the face of UN sanctions and possible US and Israeli military action.

Mr ElBaradei also previewed the IAEA findings on a site in Syria bombed in 2007 by Israel, and alleged by Israel and the US to be a nuclear reactor under construction. He said samples taken at the site were not conclusive and called on Damascus to offer more co-operation.

Jane's Intelligence Review reported this week that satellite imagery between 2005 and 2008 showed "significant levels of construction" at an alleged chemical weapons facility at Al Safir, in north-western Syria.

Any suggestion that Syria is enhancing the offensive potential of its chemical munitions will be treated as an additional threat to Israel's existence by its defence establishment.

Syria has maintained stockpiles of chemical weapons, including sarin gas and blister agents, for decades. But satellite images from two operators, GeoEye and DigitalGlobe, appeared to show efforts to update known facilities.

The Jane's report said structures for warehousing and manufacturing complex chemical materials had been built. The buildings had sophisticated filtration systems and cooling towers. Bays for specially adapted Scud missiles had also been built.

An analysis by Jane's suggested the work on the al-Safir facility had started in 2005, in the aftermath of the Iraq war, and was continuing last year.

The images show that al-Safir is protected by modern surface-to-air missiles acquired from Russia and several rings of security.

Syria's President Bashar al-Assad this week spoke about the prospect of normalising relations with the US. He said: "We are still in the period of gestures and signals. There is nothing real yet."