Iran has begun putting down piping and electric cables for its underground uranium enrichment plant, diplomats said Friday, enlisting hundreds of workers in an effort to move ahead quickly with a program that can be misused to make nuclear arms.

The move marks an escalation of the confrontation between Tehran and the world's major powers over the Islamic republic's nuclear program and will likely spur U.S. efforts to sharpen existing U.N. sanctions slapped on Iran for its defiance of a Security Council demand that it freeze enrichment efforts.

Iran says it wants to develop enrichment to generate power, but the United States and other countries fear Tehran will use the material for the fissile core of nuclear warheads.

A diplomat accredited to the International Atomic Energy Agency, which monitors Iran's nuclear program, said hundreds of technicians and laborers "were working feverishly" at the Natanz underground facility, laying down pipes and wiring needed for the centrifuges that spin uranium into enriched levels.

The diplomat was one of three who spoke to The Associated Press on condition of anonymity because the information is confidential.

The diplomats emphasized that the work at the plant was preliminary. One said centrifuges were being lowered by freight elevator into the facility, along with other equipment needed to assemble "cascades" — centrifuges in series that spin and re-spin uranium gas to the required level of enrichment. They said to their knowledge, no centrifuges had been set up by Thursday.

Less critical preparatory work inside the Natanz facility — set underground to protect it from air attack — had been going on for weeks, said one of the diplomats, adding that it was unclear when the assembling of piping and cabling that will link the centrifuges had begun.

The work appeared to back statements from both the IAEA and the Iranian leadership saying the actual setup of centrifuges at the underground site would begin this month. Iranian President Mahmoud Ahmadinejad suggested Thursday that hookups would start next week, with the aim of linking 3,000 of the machines.

Even if Tehran proves successful in installing 3,000 centrifuges, experts estimate that it would take Tehran several years for all of them to be running smoothly and without breakdowns. Once that happens, Tehran could produce two bombs a year.

Ultimately, Iran plans to have 54,000 centrifuges producing enriched uranium.

On Wednesday, the London-based International Institute for Strategic Studies think tank estimated that Iran is two to three years away from having the capacity to build a nuclear weapon.

The head of national intelligence for the U.S., John Negroponte, has spoken of a four-year time frame, and IAEA chief Mohamed ElBaradei frequently cites Negroponte when asked how long it would take Tehran to build such a weapon.

While Iran could conceivably build a bomb in two years, a three-year time frame was more likely, said Mark Fitzpatrick, a nonproliferation expert at the institute. He said estimates floated by U.S. intelligence were conservative — a likely result of its chastening experience in Iraq.

Iran now has two experimental cascades of 164 centrifuges each, as well as several partially assembled cascades, all above ground at Natanz. They have been the subject or regular inspections by IAEA teams, although their authority has been restricted for a year, since Tehran withdrew broader inspecting rights after its nuclear file was referred to the U.N. Security Council.

So far, its two linked chains of 164 machines have been operating sporadically at the aboveground portion of the Natanz facility, producing small quantities of non-weapons grade enriched uranium, IAEA inspectors say.

Two smaller assemblies underground have been "dry testing" — without gas — since November, they say.

Ahmadinejad's remarks Thursday signaled that Iran would begin the installation before Feb. 11 — the final day of nationwide celebrations in memory of the Islamic revolution. He also has called people to the streets that day to show support for the nuclear program.

"Enemies of the Iranian nation ... must know that their wrongful beliefs will be revealed once again during Feb. 11 rallies by the great Iranian nation," he said, according to the state-run news agency.

The Security Council — which last month agreed on limited sanctions targeting people and programs linked to Iran's nuclear and missile programs — has threatened to impose further sanctions on Iran later this month if it continues to refuse to roll back its program.