Turkey defiant as US threatens Ankara over S-400 missiles.

US Vice President Mike Pence warned Turkey on Wednesday that it could risk its membership in NATO if it goes ahead with plans to buy a Russian air defense system despite widespread international opposition.

Pence’s escalating rhetoric came after Turkey insisted on Wednesday that the Russian deal was done, signaling an apparent impasse between the two NATO allies.

Speaking at a NATO 70th anniversary event in Brussels, Pence said Turkey risks expulsion from the F-35 Joint Strike Fighter (JSF) programme, which could cripple Turkish manufacturers who are making parts for the aircraft. And he raised the possibility of much broader repercussions.

“Turkey must choose,” Pence said. “Does it want to remain a critical partner in the most successful military alliance in the history of the world or does it want to risk the security of that partnership by making reckless decisions that undermine our alliance?”

Turkey’s Vice President Fuad Oktay replied on Twitter saying that “The United States must choose” about whether to remain allied with Ankara. He raised the contentious relationship of US support for Kurdish forces in Syria, accusing Washington of “joining forces with terrorists”.

Other US officials have stopped short of threatening Turkey’s underlying relationship with the US or NATO over the S-400 purchase, but there have been growing calls for swift action to try to stop the deal, including possible sanctions against Ankara.

Turkey’s foreign minister, however, appeared unmoved by the US threats, saying that buying the Russian S-400 system “is a done deal. We will not step back from this.”

The US has offered to sell Turkey the American-made Patriot missile defense system, but Mevlut Cavusoglu said it could not be delivered in time.

“We couldn’t get it for 10 years,” Cavusoglu said at the Atlantic Council on Wednesday morning. “That’s why we had to buy from Russia.

“And we tried to buy from other allies as well. It didn’t work. So it is an urgent need for Turkey. I mean, we need air defense systems urgently in Turkey.”

Cavusoglu also met Wednesday with US Secretary of State Mike Pompeo and leaders of the House of Representatives Foreign Affairs Committee, whose Democratic chairman and ranking Republican member said they “made clear our bipartisan opposition” to Turkey’s plan to acquire the Russian missile system.

Republican Congressmen Eliot Engel and Michael McCaul said they cautioned Cavusoglu that deepening ties with Moscow “will risk sanctions and Turkey’s involvement in the F-35 programme”.

The US and other NATO allies have repeatedly complained about the purchase of the S-400 system, saying it is not compatible with other allied systems and would represent a threat to the F-35 stealth aircraft.

Pence described the purchase as “deeply troubling” and said it “poses a great danger to NATO, to the strength of the alliance.”

As a result, US government leaders have repeatedly threatened to shut down Turkey’s participation in the F-35 programme if the Russian deal goes through.

On Monday, the Pentagon took its first retaliatory step and stopped the delivery of F-35 fighter jet parts and manuals to Turkey.

However, so far, Turkish pilots continue to train on the aircraft at Luke Air Force Base, outside Phoenix, Arizona. Pentagon leaders say there is still hope for some type of negotiated settlement.

Acting Defense Secretary Patrick Shanahan also expressed optimism, saying he is confident that the Patriot system proposal will work out.

Cavusoglu, however, said he believes Turkey can convince the US that the S-400 will not be a risk or threat to NATO.

“We made very clear that this system will not see any NATO system, including F-35s, as an enemy,” he said, adding that Turkey has proposed setting up a technical working group that would ensure that the S-400 did not pose a threat.