One burning question in the global effort to prevent Iran from becoming a nuclear power is this: How long would it take for the Islamic Republic to build a bomb?

U.S. Secretary of State John Kerry says up to six years. Others say two or three. The answer is important because time is a critical asset in formulating a global response to an Iranian nuclear threat.

The U.S. administration has dismissed Israeli Prime Minister Benjamin Netanyahu's speech Tuesday to Congress urging legislators to oppose the agreement being negotiated in the Swiss resort town of Montreux. It says the Israeli leader offered no alternatives and the emerging pact is the best way to give the world enough time to react, should Tehran try to build a bomb.

Still, the debate over time is understandable. Views differ on whether Iran wants to make nuclear weapons, and if so, how far into the game it is.

Tehran has much of the enabling technology but says it is not interested in such arms. The U.S. and its allies say it has not decided to make them but could do so. And the U.N's International Atomic Energy Agency says it has evidence pointing to past work on such weaponry by Iran — but cannot say for sure how far it has advanced.

Such variables reflect the difficulties in gauging how much time the world would have to react, if Tehran did opt to manufacture an atomic weapon.

Ahead of a late-March deadline, the U.S. and five other world powers appear to be narrowing differences with Iran on one potential bomb-making technology — uranium enrichment. In urging critics in Congress to give negotiations a chance, the U.S. administration says it will not accept a pact unless it stretches to at least a year the time Iran would need to produce enough material for the fissile core of a nuclear weapon.

Having enough weapons-grade enriched uranium isn't enough, however.

Iran also would need to design a warhead and be able to mount it on a missile reliable and powerful enough to deliver it. Tehran has a ways to go to be able to do that, and senior U.S. officials are seizing on the extra time that gives the world to react, as they try to dissuade congressional sceptics from pursuing deal-breaking new sanctions against Tehran.

Kerry told Congress last week that Tehran is "four, five, six years" away from that capacity even after hypothetical generation of the 25 kilograms (60 pounds) of weapons-grade uranium needed for one warhead. Experts, however say that is too optimistic, although they agree that Iran is not thought to now be able to make a working nuclear weapon.

"Six years is a bit over the top for a missile-delivered nuclear warhead, and this claim contradicts U.S. intelligence assessments," says David Albright, whose Washington-based Institute for Science and Security is consulted by the U.S. government. "I think Kerry is going to extreme estimates to make a political point, not an accurate technical one."

President Barack Obama referred two years ago to intelligence evaluations as saying "it would take Iran over a year or so" to develop a nuclear weapon after a decision to do so. A year earlier, then Secretary of Defense Leon Panetta said Tehran would need "an additional one to two years to develop a deliver vehicle" for such a weapon.

U.S. intelligence chief James Clapper says that Iran "does not face any insurmountable technical barriers to producing a nuclear weapon." But he gave no gave no estimate of how long that would take, in his annual global threat assessment report published last week.

Sequence also is important. If Iran ditched any deal, it would be unlikely to wait until it had enough weapons grade uranium before fully working out on how to deliver it.

"Historically, nuclear-weapon design and even manufacturing have gone hand by hand with the production of fissile material," says Olli Heinonen, a former deputy director general of the International Atomic Energy Agency. "The bomb to Hiroshima was dropped without testing only a few weeks after the last chunks of high enriched uranium were produced."

It was under Heinonen that the IAEA went public more than three years ago with detailed and "credible" allegations of secret Iranian work on "activities related to the development of a nuclear payload for a missile."

Iran says the suspicions are based on forged U.S. and Israeli intelligence, and their backers point to recent revelations that the CIA supplied Tehran with bogus documents to disrupt any bomb-making ability. But IAEA attempts to fully investigate the allegations have been essentially stalemated for more than a decade. That means that the agency can neither confirm nor dismiss them, nor make an accurate estimate of how long Tehran might need to make such a weapon.

Tehran's missile program also is part of any equation.

The 2011 IAEA report alleged that Iran had clandestinely worked on re-engineering a medium-range Shahab 3-missile to fit a nuclear warhead. And Clapper, in his Thursday report, said Tehran would likely use missiles to deliver any nuclear weapon.

"Iran's ballistic missiles are inherently capable of delivering WMD" — weapons of mass destruction — the report said. It added that Iran is likely interested in developing intercontinental weapons.

The U.S. insists that any Iran nuclear deal must constrain further missile work by Tehran. But the Islamic Republic says no. And a diplomat familiar with the negotiations said that — with only weeks to go before the March deadline — there have been no serious negotiations on a missile moratorium.

But Iran may not even need to develop its missiles. North Korea effectively joined the club of nuclear weapon states nine years ago with its first atomic test, giving it some of the global clout it craved.

"The time to prepare a test device is a fraction of the time to build a missile deliverable one," Albright says. That time, he says, "is measured in months in the case of Iran."