Turkey and Russia are close to signing a contract for supplying of S-400 missile systems, Russian defense giant Rosoboronexport said Tuesday.

"The contract is close to signing, but has not been signed yet and, accordingly, has not yet entered into force," Rosoboronexport general director Alexander Miheyev said, adding that "the work continues, technical nuances remain."

Turkey has agreed to pay Russia $2.5 billion for the S-400 systems, a Turkish official said July 13, making the country the first NATO member country to purchase the S-400 defense system.

With the purchase of the S-400's, 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 S-400 deal also involves transfer of technology and know-how.

The S-400 missiles, which were introduced in 2007, are 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.

The defense system is able to carry missiles, which has the capability to destroy ground and air targets, including ballistic and cruise missiles.