Vladimir Kojin, an aide to Russian President Vladimir Putin, has said S-400 missile defense systems are expected to be delivered to Turkey in mid-2019 or early 2020, as the two sides have worked out the finer details of the agreement.

In September, Turkey agreed to buy two S-400s – Russia's most advanced long-range anti-aircraft missile system – in a deal worth over $2 billion. Officials from Ankara have repeatedly said that the purchase of the S-400 systems was prompted to fulfill the country's security needs.

"All the issues have been resolved and I discussed this while literally entering this hall," Kojin said, according to Russia's official Tass news agency.

Earlier this week, Foreign Minister Mevlüt Çavuşoğlu said there are no problems with the deal.

"The contract is entering the implementation stage in the coming days. A part of the contract will be paid for by the Turkish side and the other part will be covered by a loan granted by the Russian side," Kojin said.

As a NATO member country, Turkey's decision to buy the Russian S-400 systems has raised concern among other NATO member countries. However, NATO Secretary-General Jens Stoltenberg ended speculations, saying member states have the sovereign right to make decisions regarding their military purchases.

In an effort to deepen military cooperation, Turkey and Russia will reportedly sign a cooperation deal prior to setting up the S-400 systems, according to the head of Russia's state-owned Rostec Defense Company, Sergey Chemezov.

The S-400 missile defense system is Russia's most advanced long-range anti-aircraft missile system and can carry three types of missiles capable of destroying targets, including ballistic and cruise missiles.

The system can track and engage up to 300 targets at a time and has an altitude ceiling of 27 kilometers. With the purchase of the S-400s, Ankara aims to build Turkey's first long-range air and anti-missile defense system to guard against threats in the region.

Moreover, Turkey seeks to build its own missile defense systems as the deal also involves the transfer of technology and knowhow. The S-400 system, which was introduced in 2007, is the new generation of Russian missile systems, and so far Russia has only sold them to China and India.

Last year, both China and India signed an intergovernmental agreement for the procurement of four regiments of Russian-made S-400s, and delivery of the new air defense systems is expected to begin in 2020.