The US State Department has expressed concern at recent reports that Iran has deployed an advanced air defense system to guard a secretive nuclear site.

On Sunday, Iranian state television claimed Tehran had stationed a recently delivered a Russian-made long-range missile system to central Iran to protect its Fordo nuclear facility, suspected to have housed nuclear arms development work.

A video showed an S-300 carrier truck in Fordo, raising its missile launchers toward the sky, next to other counter-strike weaponry.

State Department spokesman John Kirby told a press briefing Monday that the US was unhappy with the sale of the S-300 system as well as its placement at Fordo.

“We’ve seen the reports of this deployment. Obviously, that’s of concern to us because we have long objected to the sale of Iran – of these kinds of capabilities,” Kirby said.

The Russian-made missile defense system is one of the most advanced of its kind in the world, offering long-range protection against both airplanes and missiles. The first shipment arrived in Iran in April.

Kirby said the US would be in contact with allies regarding the deployment of the battery.

“As we get more information, obviously, we’re going to stay in close consultation with partners going forward,” he added, without giving more details.

Kirby admitted that the question of the S-300 missiles was not a prominent part of last Friday’s meeting in Geneva between US Secretary of State John Kerry and his Russian counterpart Sergey Lavrov.

In 2010 Russia froze a deal to supply the missile system to Iran, linking the decision to UN sanctions instituted because of Tehran’s nuclear program. Russian President Vladimir Putin lifted the suspension in July 2015, following Iran’s deal with six world powers that curbed its nuclear program in exchange for relief from international sanctions.

A satellite image of Iran's Fordo uranium enrichment facility (photo credit: AP/DigitalGlobe)

A satellite image of Iran’s Fordo uranium enrichment facility. (AP/DigitalGlobe)

Israel has long sought to block the sale to Iran of the S-300 system, which analysts say could impede a potential Israeli strike on Tehran’s nuclear facilities. Other officials have expressed concern that the systems could reach Syria and Hezbollah, diluting Israel’s regional air supremacy.

The Fordo site, built into a mountain near the city of Qom, has stopped enriching uranium since the January implementation of the nuclear deal. US officials had suggested the heavily fortified site was being used to produce weapons-grade uranium.

The site was considered to be a main target should Israel or another country try to launch airstrikes to set back Iran’s nuclear program.

AFP and Times of Israel staff contributed to this report.