"This White House understands that -- working with our allies -- that this is an urgent problem that has to be addressed and we can't delay addressing," spokesman Robert Gibbs told reporters.

The comments came a day after International Atomic Energy Agency said Iran was continuing to enrich uranium, a key stage in the atomic bomb-making process, but had slowed down the expansion of its enrichment activities.

Washington also expressed concern about Tehran-ally Syria's atomic ambitions, and summoned Damascus's ambassador in Washington to discuss IAEA findings of unexplained uranium particles at a remote desert site.

The US envoy to the United Nations, Susan Rice, meanwhile said in a radio interview that the IAEA report "confirms what we all have feared and anticipated, which is that Iran ... remains in pursuit of its nuclear program."

"There's no ambiguity about that, and our aim is to combine enhanced pressures, and indeed the potential for direct engagement to try to prevent Iran from taking its program to fruition," Rice said in remarks to be aired later Friday on National Public Radio's All Things Considered program.

The report from the UN nuclear watchdog conceded that, despite six years of intensive investigation, it was no closer to determining whether Iran's disputed nuclear drive is as peaceful as Tehran claims.

"The report represents another lost opportunity for Iran as it continues to renege on its international obligations. Absent compliance, the international community cannot have confidence that this program is exclusively of a peaceful nature," said Gibbs.

"It does underscore the urgency with which the international community must work together to address these enrichment activities," said the spokesman.

After UN inspectors also found further uranium particles, as well as traces of graphite at Syria's Al-Kibar according an IAEA report, the State Department "requested a meeting" with Syria's Ambassador to Washington, Imad Moustapha.

Spokesman Gordon Duguid said US officials have asked to meet with Moustapha "to discuss our concerns."

There remain "key differences between our two governments including concerns about Syrian support to terrorist groups and networks and Syria's pursuit of non-conventional nuclear weapons," he added.

On Thursday, Washington urged the IAEA to discuss what it said was mounting evidence of a clandestine nuclear program in Syria at a meeting next month in Vienna.

In a report obtained by AFP, the IAEA rejected assertions by Damascus that particles of uranium found at the remote Al-Kibar site came from Israeli missiles used to bomb it in September 2007.

"It's nuclear material that hasn't been declared and Syria has to explain" how it got there, said a senior IAEA official, speaking on condition of anonymity.

Syria insists Al-Kibar is a disused military facility, and that any uranium there could have been from the Israeli bombs.

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