Iran is not making fast progress towards acquiring a nuclear weapon, a US expert said Friday, adding he believed Tehran would still need another two years to achieve that goal.

"Iran is not moving as fast as it could. They've been at it since 25 years since they started the Iranian enrichment program in about 1985," said Mark Fitzpatrick, from the International Institute for Strategic Studies.

He said Iran would still need "a little over two years to have a bomb."

Fitzpatrick also compared Tehran's slow progress to the 11 years it took Pakistan to acquire a nuclear capacity, as he presented an IISS report entitled "Iran's nuclear, chemical and biological capabilities: a net assessment."

But Fitzpatrick, a former State Department employee, added Iran had still not yet completely decided whether to press ahead with making a nuclear bomb.

"As long they haven't made that decision I think there is still a time for diplomacy," he said.

At the end of December, Israeli Foreign Minister Moshe Yaalon said several recent setbacks had delayed Tehran's acquisition of a nuclear capability.

One of them was the Internet virus, the Stuxnet worm, which some suspect was developed by Israel and the United States and which affected the Iranian centrifuges producing enriched uranium -- a vital component of a nuclear bomb.

The New York Times reported in January that US and Israeli intelligence services collaborated to develop the computer worm.

"Stuxnet has had an impact on putting some centrifugues out of operation. But it was not a complete success because they were able to operate," Fitzpatrick said.

UN sanctions against the Islamic Republic have also impacted the alleged Iranian nuclear program, Washington has said.

The Sajil 2 missile -- which would be used to carry a nuclear warhead -- was also "still two years away from being operational," Fitzpatrick said.

But the IISS report said Iran's nuclear program has been making inexorable progress in the past 25 years, and argued that the Iranian regime's insistence that it was for peaceful civilian purposes only were simply not credible.

Iran has been slapped with four sets of UN sanctions for refusing to rein in its suspect nuclear program and for failing to halt uranium enrichment, amid accusations from the United States and other western nations that it is seeking to develop an atomic bomb.

Tehran has steadfastly denied the allegations.

An influential US senator said last week 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.