Russia has provided Iran with the first batch of S-300 air defense systems, Russian Presidential Aide Vladimir Kozhin said Thursday.

“We have a signed contract with Iran, the first batch has now been unloaded, approximately a battalion. We will continue to fulfill our commitments until year-end,” Kozhin said.

Russia and Iran initially signed an S-300 delivery deal in 2007, but its implementation was halted by Moscow due to UN Security Council sanctions against Tehran. In response, Iran filed a lawsuit against Russia.

The S-300 is a series of highly capable, long-range surface-to-air missile complexes first deployed in the USSR in 1979 and later modified by the Russian Armed Forces.

As well as targeting aircraft, the fully mobile units have the capacity to engage ballistic missiles.

Experts believe that Russia is most likely delivering the S-300PMU-2 systems - also known by the NATO codename SA-20 - which were first introduced in 1997 and are comparable to the US Patriot Air and Missile Defense System.

The Russian batteries are made by a state-run company, Almaz-Antey, who announced in 2012 it was stopping the production of the S-300s and switching resources to the more advanced S-400s.