Turkey's recent acquisition of a Russian S-400 missile defence systems is causing "a great deal of complication" within Nato, the US secretary of the army told The National on Wednesday.
Ryan McCarthy, who is visiting European allies this week to discuss joint training missions and collaboration on weapons systems, said he saw greater sophistication and scale in Russian military capabilities.
“There has been a considerable increase in the Russian army and weapons systems,” Mr McCarthy said.
Mr McCarthy said discussions with Ankara were continuing after Turkey acquired the S-400 system a year ago, despite objections from Nato and Washington.
“The S-400 procurement brings a great deal of complication to the relationship,” he said.
Mr McCarthy stressed a diplomatic resolution to the issue but said the US administration continued to consider other options.
Last week, a bipartisan group of senators wrote a letter to US Secretary of Defence Mark Esper, urging him “to remove Turkey from the supply chain of the F-35 Lightning II Joint Strike Fighter”, after its acquisition of the S-400 and other matters.
“From human rights violations in Syria to arbitrary arrests of Americans in Ankara, to defence co-operation with Russia, Turkey is not behaving like a responsible actor or working collaboratively with the West at the level we expect from a Nato ally,” the letter said.
It was signed by Republican Senators James Lankford and Thom Tillis, and Democratic Senators Jeanne Shaheen and Chris Van Hollen.
They said that continuing the contracts for the new jet, which is the most expensive weapons system yet created, hurts US geopolitical efforts.
"By keeping Turkish manufacturers in the supply chain two years after the initial statute took effect and well beyond the Pentagon’s self-imposed deadline, the Department is impeding our nation’s diplomatic and geopolitical efforts to pressure Turkey to reverse course,” the senators wrote.
Turkey has not been dissuaded from its acquisition of the S-400. It tested the Russian system against US F-16 jets last November.
Ankara could face more penalties under the Countering America’s Adversaries through Sanctions Act that passed overwhelmingly in 2017.
It was designed to sanction anyone involved in significant transactions with Russia.
US President Donald Trump has been delaying the imposition of such sanctions on Turkey, a Nato ally.
Mr Trump has fostered a close relationship with Turkish President Recep Tayyip Erdogan, who he called on Tuesday to discussed trade negotiations, the White House said.
Mr McCarthy told The National that the accusations of Russian bounties paid to the Taliban in Afghanistan to kill coalition troops did not come up during his talks with the Europeans.