U.S. VP Pence warns Turkey against buying Russian air defenses

"We will not step back", he said, asserting, "Turkey doesn't have to choose between Russian Federation and any others".

"Pending an unequivocal Turkish decision to forgo delivery of the S-400, deliveries and activities associated with the stand-up of Turkey's F-35 operational capability have been suspended", Air Force Lieutenant-Colonel Mike Andrews, a Defense Department spokesman, said in a statement, as first reported by Reuters.

"We don't see our relations with Russian Federation as an alternative to our relations with others and nobody, neither West nor Russian Federation, should or can ask us to choose between", he added.

In a PBS Newshour interview broadcast on Wednesday, Cavusoglu said Ankara proposed "to form a technical working group and to examine" any United States concerns over the S-400s.

Washington and Ankara are at odds over issues including U.S. support for a Syrian Kurdish militia viewed as terrorists by Ankara and Turkey´s purchase of a Russian missile system over U.S. objections.

Turkey has insisted it will operate the S-400 as a standalone system and will not hook it up to the same network as the F-35. Russian Federation will deliver the first batch of the defence batteries in late 2019.

But so far, Turkish pilots continue to train on the aircraft at Luke Air Force Base, outside Phoenix, Arizona.

Hours before foreign ministers from the 29-member Western alliance opened talks in Washington, Vice President Mike Pence delivered a stinging rebuke both to Germany over its level of defense spending and to Turkey for buying a major arms system from Russian Federation.

Mr Pence also rebuked Germany - another key North Atlantic Treaty Organisation member - for not spending enough on its defence. The Turkish lira fell by 2% amid news of the USA move to halt equipment deliveries and the ruling party's setbacks in local elections.

In an attempt to persuade Turkey to drop its plans to buy the S-400, the United States offered the pricier American-made Patriot anti-missile system in a discounted deal that expired at the end of March.

"And even President Trump admitted on the phone that it was the mistake of the previous administration of United States that they couldn't sell Patriots to us".

"Turkey was on board the F-35 program from day one", says Loren Thompson, CEO of the Lexington Institute, a public policy think tank that receives some of its funding from F-35 manufacturer Lockheed Martin.

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

The Turkish military completed preparations for a cross border operation to the region east of the Euphrates River to clear PKK-affiliated terrorists, which have been posing a national security threat for Ankara, out of northern Syria.

Turkey expressed irritation with the State Department Wednesday over its description of a meeting between Secretary of State Mike Pompeo and Turkish Foreign Minister Çavuşoğlu earlier this week.