Iran announced that they have deployed the Russian-built S-300 surface to air missile system around its nuclear enrichment facility at Fordow. 

The Russians initially sold the system to Iran in 2010, but backed down under pressure from the US and western powers. Following the interim nuclear agreement with Iran in April of last year, the Russians agreed to supply Iran with the sophisticated air defense system.

Reuters:

"Our main priority is to protect Iran's nuclear facilities under any circumstances," Brigadier General Farzad Esmaili, commander of the Islamic Revolutionary Guards Corps' (IRGC) air defense force told state TV.

Iran and the six major powers reached a landmark nuclear deal in 2015 aimed at curbing Iran's nuclear programme in exchange for lifting nuclear-related sanctions imposed on Tehran over its disputed nuclear work.

Enrichment of uranium at the Fordow facility, around 100 km (60 miles) south of Tehran, has stopped since the implementation of the nuclear deal in January.

Russia, under pressure from the West, in 2010 canceled a contract to deliver S-300s to Iran. But Russian President Vladimir Putin lifted that self-imposed ban in April 2015, after an interim deal was reached between Iran and the six powers.

In August, Iran said that Russia had delivered main parts of the system to the country, adding that the missile system would be completely delivered by the end of 2016.

The IRGC's Esmaili did not say whether the system was operational, but added: "Today, Iran's sky is one of the most secure in the Middle East".

The S-300 was first deployed by the Soviet Union in 1979, but has undergone several upgrades since then. It's unclear what version the Russians sold the Iranians, but just about anything would be an improvement over Iran's aging air defense missile systems it purchased from the US, China, and the Russians.

It is significant that Iran has deployed the asset to protect the Fordow facility. Their main enrichment efforts are located at at the Nantanz facility. Fodrow was built in secrecy, only discovered by the west in 2007 and not declared by Iran to the IAEA until 2009. It is an underground facility, carved inside a mountain, whose 3,000 centrifuges enrich uranium from 3% to 20%. 

If Iran were going to enrich uranium to the 90% level to build a bomb, Fordow would be the place it would happen.

The S-300 is a substantial upgrade for Iran and would make it more difficult to attack Fordow and any other sites it protects.