SALZBURG, Austria — Iran is poised to double its output of higher-enriched uranium at its fortified underground facility, the U.N. nuclear agency said Friday - a development that puts Tehran within months of being able to make the core of a nuclear warhead.

In its report, the International Atomic Energy Agency said Iran was ready within days to ramp up its production of 20 percent enriched uranium at its plant at Fordo using 700 more centrifuges.

That would double Iran’s present output and cut in half the time it would take to acquire enough of the substance needed to make a nuclear weapon, reducing it to just more than three months.

Iran says it has no interest in making nuclear arms, just nuclear power for its citizens, but the United States and other nations believe otherwise. Iran has refused to give up enrichment despite international sanctions and offers of reactor fuel from abroad and for years has stalemated an agency probe of suspicions that it worked secretly on developing such arms.

The report urged Iran to stop stalling the probe, declaring that unless it starts to cooperate the agency cannot “exclude the existence of possible military dimensions to Iran’s nuclear program.”

The report also clashed with comments by Israeli officials suggesting that Iran has slowed the timetable for reaching the ability to makenuclear weapons.

The discrepancy is important because the earlier Israeli comments implied that Israel would have more time before deciding whether to hit Iranian facilities in an attempt to slow Tehran’s perceived efforts to make nuclear weapons.

The report was circulated among the agency’s 35 board member states. It said between the last board report in August and now, Iran had put nearly 700 centrifuges that were installed but not ready to operate at Fordo under a vacuum to make sure they are airtight.

That is the last step before uranium gas is fed into the centrifuges and the process or enrichment begins - an activity that can produce both reactor fuel or at high levels the fissile interior of a nuclear weapon. It takes only a few days to start enrichment with machines that are under vacuum.

The centrifuges, “having been subjected to vacuum testing, were ready for feeding” with uranium gas, the report said.

About 700 other centrifuges have already been producing 20 percent uranium at Fordo since early this year. Another 1,400 or so have been installed but are not yet believed operational - bringing the total to about 2,800 in all.

While experts agree that the Islamic Republic could assemble enough weapons-grade uranium to arm a nuclear weapon relatively quickly, they point out that this is only one of a series of steps needed to create a working weapon.

They say that Tehran is believed to be years away from mastering the technology to manufacture a fully operational warhead.

Earlier this month, Israeli Defense Minister Ehud Barak said Iran has “essentially delayed their arrival at the red line by eight months.” That jibes with the time frame laid out by Israeli Prime Minister Benjamin Netanyahu in September, when he spoke at the U.N. General Assembly. There, Netanyahu said the world has until next summer at the latest to stop Iran before it can build a nuclear bomb.

Iran has a far larger enrichment plant at Natanz, in central Iran, which churns out uranium enriched below 4 percent. But the 20 percent material being produced at Fordo is of greater concern to the international community because it can be turned into weapons-grade uranium of 90 percent purity much more simply and quickly - and because the facility, near the holy city of Qom, is well protected against attack.

About 300 pounds of weapons-grade uranium is neededfor at least one warhead.

Olli Heinonen, who headed the agency’s Iran probe until 2010, said Tehran would likely be able to produce enough 20 percent enriched uranium by the summer to be able to make weapons-grade uranium for two or three warheads, if it doubles its Fordo capacity.

The report’s other main area of concern focused on Parchin, a military site southeast of Tehran that the agency suspects was used for testing high explosives used to set off a nuclear charge - and which the agency fears is undergoing a major cleanup.

Iran has repeatedly turned down requests to visit the site, which, the report warned, “seriously undermined” the agency’s ability to conduct effective verification. The report cited satellite photos showing that Iran has torn down buildings, dug up awide area around the suspect site and embarked on other suspect activities indicating sanitization attempts there.

In Washington, State Department spokesman Victoria Nuland would not commenton the report but said the agency been calling on Iran to intensify efforts toward an agreement on its nuclear program for more than a year.

“That has not happened,” she said.

Front Section, Pages 7 on 11/17/2012