An influential US Senator said Wednesday after a closed-door, classified intelligence briefing on Iran that Tehran is working "seriously" to develop nuclear weapons.

"I can't say much in detail, but it's pretty clear that they're continuing to work seriously on a nuclear weapons program," Independent Senator Joe Lieberman, who chairs the Senate Homeland Security committee, told AFP.

The lawmaker, who also sits on the Senate Armed Services Committee, spoke after a briefing from a senior US intelligence official on weapons of mass destruction on the latest US National Intelligence Estimate (NIE) on Iran.

A previous NIE on Iran, partly declassified in December 2007, stated with "high confidence" that Tehran had "halted its nuclear weapons program" in late 2003. The document is the consensus view of all 16 US spy agencies.

In February, a US official told AFP on condition of anonymity that US intelligence agencies believe Iran's leaders are locked in debate about whether to build nuclear weapons and that sanctions have aggravated those divisions.

The official said the assessment was detailed in the latest NIE on Iran, which was the subject of Wednesday's closed-door briefing.

Lieberman, after noting that he was speaking in general terms, said that Iranian activities "pose a threat" on a range of fronts, from its suspect atomic drive to its activities in Iran and Afghanistan.

"We know their intentions toward us from what they say, which is hostile, and from what they have done in Iraq, which is to train a lot of Shia extremists who've killed hundreds of American soldiers, and what they're beginning to do more of in Afghanistan," he said.

The senators heard from a top weapons of mass destruction and proliferation official on the US National Intelligence Council that reports to Director of National Intelligence (DNI) James Clapper.

Lawmakers were tight-lipped as they emerged from the session, with most declining to comment even in general terms on the Islamic republic's suspect nuclear program -- a frequent target of angry denunciations in the US Congress.

Senate Armed Services Committee Chairman Carl Levin, a Democrat who organized the session, simply said he "learned a lot."