Iran's hard-line regime has reversed its position in the ongoing nuclear showdown with the West, agreeing in the last week to allow United Nations inspectors into two significant nuclear facilities, diplomats tell FOX News.

United Nations inspectors have been granted access to Iran's heavy water facility at Arak, as well as the enrichment facility at Natanz — the site of Iran's primary centrifuge installation.

Iran's decision to allow inspectors into the sites is "a long awaited breakthrough," a diplomatic source tells FOX News.

"The Iranians loosened up a bit in the last week, allowing the inspectors in," the source said.

Arak houses a nearly completed nuclear reactor. The site produces material for nuclear fuel that can be further enriched to provide fissile material for warheads. The past week's visit marks the first time in a year that international inspectors have been granted access to visit the heavy water reactor.

Additionally, diplomats have been granted greater monitoring rights at the site in Natanz, the location of centrifuge production and installation.

Since its clandestine enrichment efforts were revealed more than six years ago, Iran has steadily increased activities at its cavernous underground facility at Natanz, about 300 miles south of Tehran, shrugging off three sets of U.N. Security Council sanctions and rejecting talks meant to entice it to mothball the activity.

A June IAEA report said nearly 5,000 centrifuges were now enriching at Natanz — about 1,000 more than at the time of the last agency report, issued in February — with more than 2,000 others ready to start enriching. A new report due in the next week or so is expected to confirm that operations continue to expand — along with Tehran's potential capacity to produce weapons-grade uranium.

Most experts estimate that the more than 1,000 kilograms of low-enriched uranium Iran had accumulated by February was already enough to produce enough weapons-grade material through further enrichment for one nuclear weapon.

Tehran says it has a right to enrich, insists it is not interested in making weapons and has no intention of reconfiguring its operations from churning out nuclear fuel-grade material to highly enriched uranium suitable for nuclear arms.

Before lifting the ban on visiting Arak, Tehran had repeatedly refused inspection requests from the International Atomic Energy Agency, despite warnings by the agency that its stance contravened mutual agreements.

Western countries have repeatedly called on Iran to stop construction of the reactor, fearing it could be used as a second track toward building a warhead. When finished, say experts, Arak could produce enough plutonium for a nuclear weapon each year.

The Associated Press contributed to this report.