U.S. VP Pence threatens Turkey not to buy S-400 air defence system from Russia

US Vice President Mike Pence upped the threat against Turkey over Ankara's plans to purchase this year an advanced air defence system from Russia, saying Washington "will not stand idly by" while a NATO member buys weapons from an adversary.

"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 that alliance," Pence said in Washington.

Earlier, Turkish Foreign Minister Mevlüt Çavuşoğlu said Ankara will not step back from its deal to buy the S-400 air defence system from Russia. The agreement with Moscow was a "done deal," he said.

The two were speaking at events hours before NATO kicks off its 70th anniversary celebrations in Washington.

US officials this week said Turkey risks triggering sanctions if it goes ahead with the S-400 purchase, in addition to being kicked out of the advanced F-35 stealth fighter jet programme.

The Pentagon also stopped the delivery of F-35-related materials to Turkey and announced it was developing alternatives in case Ankara was removed from the supply chain of the advanced jet's production.

Çavuşoğlu was due to meet US Secretary of State Mike Pompeo for a one-on-one discussion during his time in Washington.

Pence said that being kicked out of the F-35 programme "will harm not just Turkey's defence capacity, but it may cripple many of the Turkish component manufacturers that supply that programme."

Çavuşoğlu said that Turkey cannot be forced to choose between the West and Russia, warning that the example of the conflict in Ukraine has shown where such things could lead.

Last week, a bipartisan group of senators introduced legislation which would block the transfer of the F-35 to Turkey unless it is certified that Ankara will not buy the Russian system, signalling opposition also on Capitol Hill.

The S-400 purchase is but one of a host of issues between the two countries, including anger in Ankara over US policy in Syria, where Washington partnered with YPG/PKK terror groups in the war-torn region.

Turkey joined the F-35 Joint Strike Fighter Program in 2002 and has invested more than $1.25 billion. It also manufactures various aircraft parts for all F-35 variants and customers.

Turkish firms supply the F-35 program with key components, including airframe structures and assemblies and the center fuselages.

Two F-35s already delivered to Turkey are currently at Luke Air Force Base in Arizona, where Turkish pilots are being trained. These jets were scheduled to be transferred to Turkey in November.

Earlier Wednesday, Turkish Foreign Minister Mevlüt Çavuşoğlu said the S-400 missile system would be independent of any NATO equipment, saying it would be Turkey's own use, and recalled the proposal to establish a working group to make sure the system would not be of any threat to the F-35s.

Çavuşoğlu said U.S. President Donald Trump said he would take care of the F-35 deal.

"Trump himself admitted on the phone that U.S. made the mistake not to sell Patriots to Turkey and he promised Erdoğan that he will take care of this issue," Çavuşoğlu said.