Iran's uranium enrichment effort has picked up speed in the past three years and has not been crippled by cyber sabotage from the Stuxnet virus, experts told US lawmakers on Wednesday.

As a result, Iran could produce enough fissile material needed for a nuclear weapon within four months, if the leadership decided to go ahead, the experts told the House Armed Services Committee.

Based on the findings of the International Atomic Energy Agency (IAEA), "it's clear that Iran could produce a nuclear weapon very quickly should it wish to do so," said Stephen Rademaker of the Bipartisan Policy Center, a Washington think tank.

Iran has produced 3,345 kilos of uranium enriched to 3.5 percent, according to the IAEA, which if it was enriched further would provide enough uranium for at least two atomic bombs, Rademaker said.

If the Iranian leadership made the decision to produce an atomic weapon, "it would take them 35 to 106 days to actually have the fissile material for a weapon," he said.

David Albright, president of the Institute for Science and International Security (ISIS), told the same hearing that "it would take Iran at least four months in order to have sufficient weapon grade uranium ... for a nuclear explosive device."

Uranium 235 must be enriched close to 90 percent for use in an atomic bomb.

The director of national intelligence, James Clapper, has said that it was "technically feasible" Tehran could produce a nuclear weapon in one or two years.

More than 9,000 Iranian centrifuges are churning out 158 kilograms of 3.5 percent enriched uranium a month, three times the production rate compared to mid-2009, when the Stuxnet virus struck the program, Rademaker said.

The enrichment rate is "three times the rate of production prior to the Stuxnet virus, which many people have suggested somehow crippled their program."

"So Stuxnet may have set them back, but not by very much, at least not sufficiently," he added.

According to The New York Times, President Barack Obama, and his predecessor in the White House, George W. Bush, approved the use of the Stuxnet virus to disrupt Iran's nuclear program, in the first known sustained US cyber attack.

Stuxnet -- a complex virus developed jointly with Israel -- sowed confusion at Iran's Natanz nuclear plant, the Times reported, but the virus later accidentally spread outside of Iran, appearing in computer systems in other countries.

Some analysts and former US military officers have touted cyber attacks as a more effective weapon against Iran's nuclear ambitions than bombing raids, which they say would carry big risks without causing permanent damage to the program.

US intelligence agencies believe Iran's leadership has not yet decided to pursue nuclear weapons, even as the regime presses ahead with uranium enrichment and other activities that could provide the capability to create an atomic arsenal.

"We really don't know if they're going to decide to build a nuclear weapon, but the indications are at least that they're on a trajectory to do so," Albright said.

If Iran decided to start producing weapons-grade uranium, either the UN nuclear watchdog or US intelligence likely would detect the move within one to three weeks, he added.

Wednesday's congressional hearing came after two days of tense talks between world powers and Iran in Moscow that failed to produce a breakthrough over Tehran's disputed nuclear program.

Iran remains opposed to any halt of its sensitive uranium enrichment work while the United States and five other major powers are not ready to lift sanctions on Tehran.

Former US senator Chuck Robb, who also testified at the House hearing, said diplomacy could only succeed if Iran believed there was a genuine threat of military force if they failed to compromise.

Robb said "it is a credible threat of force when pursued together with diplomacy and sanctions that proves the best hope for peace."