Iran has installed many more uranium enrichment machines in an underground bunker, diplomatic sources said on Thursday, potentially paving the way for a significant expansion of work the West fears is ultimately aimed at making nuclear bombs.

Iran denies allegations it is seeking a nuclear weapons capability. But its refusal to curb its nuclear enrichment program has prompted tough Western sanctions and has heightened speculation that Israel may attack its atomic sites.

In a possible sign of further Iranian defiance in the face of such pressure, several sources said Iran had put in place additional enrichment centrifuges in its Fordo facility, buried deep inside a mountain to protect it against enemy strikes.

One source suggested hundreds of machines had been installed.

The International Atomic Energy Agency was expected to press Iran again in talks on Friday for access to the site at Parchin, as part of its long-stalled probe into suspected nuclear weapons research in the Islamic state. Sources say that satellite imagery indicates that Iran has covered a building at the military site, which U.N. inspectors want to visit with a brightly-colored, tent-like structure.

Western diplomats have said they believe Iran is cleansing the Parchin site to remove any evidence of illicit nuclear activity at a place where the U.N. nuclear watchdog suspects it has conducted tests that had a military dimension.

Covering the building in question – which is believed to house a steel chamber for explosives experiments – may allow Iran to carry out sanitization or other work there which would not be seen via satellite pictures.

One Western envoy said that the suspected clean-up at Parchin was "intensifying" and that this made it doubtful that inspectors from the IAEA would uncover any hard evidence there, even if they were allowed to go.

"Given the extent of the clean-up, it is indeed unlikely the agency, if it ever gets access, would find anything at Parchin," the diplomat said.

Another diplomatic source said Iran appeared to have completed installing two more cascades – interlocked networks of 174 centrifuges each – at Fordo.

If confirmed in a new IAEA report on Iran due next week, the two alleged new cascades would be in addition to six cascades that were previously installed, of which four were refining uranium. It was unclear when Iran may launch the new machines.

Iran has previously dismissed the allegations about Parchin, which it says is a conventional military facility, as "ridiculous."

Friday's meeting, the first since previous discussions ended in failure in June, takes place after an upsurge in rhetoric from Israeli politicians this month suggesting Israel might attack Iran ahead of the U.S. presidential election in November.

A senior Iranian envoy said he expected progress at talks on Friday with the U.N. nuclear watchdog about the Islamic state's atomic activities. "We are determined to go to a ... positive conclusion," Ali Asghar Soltanieh, Iran's ambassador to the IAEA, told reporters. "Both sides are trying to bridge the gap."

He was speaking outside's Iran's diplomatic mission in Vienna shortly before the start of his talks with U.N. inspectors.

Israeli officials have already warned that an expansion of enrichment activities at Fordo will shorten the time it takes for Iran to reach a "zone of immunity" – the point after which it would become impossible for Israel to stop Iran from building a nuclear bomb as the Iranians would already have heavily fortified underground nuclear sites that may be immune to air strikes.

The IAEA is reportedly forming a special Iran team, drawing together sleuths in weapons technology, intelligence analysis, radiation and other fields of expertise as it seeks to add muscle to a probe of suspicions that Tehran worked secretly on atomic arms, diplomats told The Associated Press.

Creating a unit focused on only one country is an unusual move for the agency, reflecting the priority the U.N. nuclear watchdog is attaching to Iran.

The four diplomats, who demanded anonymity because they were not authorized to discuss the restructuring plans, spoke with AP ahead of Friday's talks. One of diplomats likened the restructuring plan to the agency's Iraq "Action Team" – the squad of experts who uncovered components of Saddam Hussein's fledgling nuclear-weapons program in the 1990s.

That unit, however, had broad on-the-ground access under U.N.-mandated inspections. That's lacking in the case of Iran, which allows agency inspectors access only to its known nuclear activities and has for years blocked its attempts to probe alleged evidence of secret nuclear weapons research and development.

Under the planned reorganization, said one of the diplomats, a "dedicated team" of about 20 experts will be drawn from the main IAEA pool to focus solely on the agency's Iran investigation. But two of the diplomats spoke of opposition by board member Russia, which has supported diplomatic efforts to nudge Iran toward a nuclear compromise while strongly opposing harsh sanctions as a means of achieving that goal.

They said senior agency officials had met recently with Russian representatives in efforts to dispel Moscow's fears that the reorganization would place too much weight on intelligence gathering – an agency function viewed with suspicion by the Russians.

Former Israel Defense Forces Chief of Staff Gabi Ashkenazi on made his first public statements on a possible Israeli attack on Iran on Wednesday evening. "Anyone who thinks that tomorrow morning we're going to wake up to a nuclear Iran – we're not there yet," Ashkenazi said at a meeting of the Council for Peace and Security.

According the Makor Rishon newspaper, Ashkenazi predicted that Israel will not attack Iran in the near future, but added, "even though we're not there yet, we're apparently on our way there."

During his speech, the former IDF chief of staff presented what he believed were three modes of operation required to stop Iran: "You have to have a covert campaign – everything that stops short of a war, brings up to the threshold of a strike. I read in the newspapers about the computer viruses or the scientists [assassinations], and I do not know anything about them, but they should continue. It only buys us time, and it is important. The second component is sanctions, both diplomatic and political but especially economic. Both of these components should support the third strategy of maintaining a credible and realistic military option, and the hope that this combination of components will dissuade Iran from trying to get a bomb."

Labor Party leader Shelly Yachimovich was meanwhile in Paris on Thursday and met with French President François Hollande. She called on France to step up pressure on Iran and "impose more severe sanctions" against its nuclear program.