Washington has reportedly relayed its concerns to Turkish officials over Ankara's move to purchase Russia's most advanced missile-defence system, the S-400. 

Turkish President Recep Tayyip Erdogan earlier this week announced that Ankara had signed a contract with Russia to buy the surface-to-air system and had paid a deposit, even though the loan arrangement was yet to be entirely worked out. It remains to be seen how Ankara’s decision to buy the Russian defence system will affect Ankara’s relations with Nato; Turkey has the second largest army in the western alliance. 

“A Nato interoperable missile-defence system remains the best option to defend Turkey from the full range of threats in its region,” Pentagon spokesman Johnny Michael said on September 12, according to Sputnik.

“Turkey continues to pursue anti-missile systems from NATO allies, including the US, for its broader, long-term missile defence needs. The United States is committed to expediting the delivery of equipment purchased by Turkey, when possible,” Michael also commented.

Russia has confirmed the arms deal worth $2.5bn, according to some reports.

“The contract has been signed and it is about to be executed,” Russian President Vladimir Putin’s aide for military-technical cooperation Vladimir Kozhin told Tass on September 12.

“I can merely guarantee that all of the decisions made under this contract strictly agree with our strategic interests. In that connection we find quite natural the reaction of some western countries that have been trying to put pressure on Turkey,” Kozhin said.

Turkey started talks with China Precision Machinery Import and Export Corp (CPMIEC) on buying a missile-defence system in September 2013, but opted not to sign the final deal after Nato allies expressed concern over Ankara’s plans to buy it, citing security and compatibility problems.

Ankara next turned to Russia to buy the anti-aircraft hardware when in 2015 it cancelled the controversial deal with the Chinese company.

With Ankara and Russia restoring ties, the talks on the missile system have gained momentum.

Regarding the situation with the loan, Erdogan noted that officials from the two countries were still working on the issue. “The process will continue by the transfer of a credit from Russia to Turkey,” Erdogan said, without providing details.

A Nato official took a somewhat softer tone than the US government spokesman, while still warning about potential incompatibility of the Russian system with those used by other Nato allies. 

“It is up to allies to decide what military they will buy. What matters for Nato is that the equipment allies acquire is able to operate together. Interoperability of our armed forces is fundamental to Nato for the conduct of our operations and missions,” Nato spokesman Mark Sanders told CNBC in an emailed statement.

No Nato ally currently operates the S-400 and Nato has not been informed about the details of any purchase, he added.

According to Nato’s policy, interoperability does not necessarily require common military equipment. What is important is that the equipment can share common facilities, and is able to interact, connect and communicate, exchange data and services with other equipment, the interoperability policy states.

The missile deal was announced with relations between Turkey and its Western allies, notably with the US and Germany, at a low point.

Erdogan is expected to meet with US President Donald Trump during the upcoming UN General Assembly in New York later this month.

Meanwhile, Berlin has announced that it has put most arms exports to Turkey on hold due to the deteriorating human rights in Turkey.

“We have put on hold all big requests that Turkey has sent to us, and these are really not a few,” German Foreign Minister Sigmar Gabriel said on September 11, according to Reuters.

Gabriel reportedly said that there were only a few exemptions such as in instances where the government’s decision is tied to international agreements or if the requested exports concerned vehicles, not weapons.

German Chancellor Angela Merkel rejected a total ban on arms exports to Turkey.

Germany will decide on arms sales requests from Turkey on a case-by-case basis, Merkel told broadcaster NDR on September 12.