U.S. Vice President Mike Pence and Speaker of the House Nancy Pelosi (D-CA) listen as NATO Secretary General Jens Stoltenberg addresses a joint meeting of Congress on Capitol Hill in Washington, U.S., April 3, 2019. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - U.S. Vice President Mike Pence warned Turkey on Wednesday against buying a Russian S-400 anti-missile system, keeping up the pressure on its NATO ally to abandon the purchase that Washington considers a threat to U.S. military equipment.

“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?” Pence said in remarks at a NATO event in Washington.

Pence’s stark warning comes as Washington and Ankara remain at loggerheads over Turkey’s plan to buy the air defense system from Russia, which Washington believes would compromise the security of its F-35 fighter jets, made by Lockheed Martin.

Turkey struck back promptly, with Turkish Vice President Fuat Oktay issuing his own warning on Twitter: “The United States must choose. Does it want to remain Turkey’s ally or risk our friendship by joining forces with terrorists to undermine its NATO ally’s defense against its enemies?”

Washington has warned that proceeding with the deal could result in U.S. sanctions and the exclusion of Turkey from the F-35 fighter jet program. The United States this week halted delivery of equipment related to the stealthy F-35 fighter aircraft to Turkey.

“We’ve also made it clear that we will not stand idly by while NATO allies purchase weapons from our adversaries that threaten the cohesion of our alliance,” Pence said.

Earlier Wednesday, Turkish Foreign Minister Mevlut Cavusoglu said Turkey has proposed to the United States that they form a working group to determine that Russian S-400 missile defense systems do not pose a threat to U.S. or NATO military equipment.

“It will not be integrated into the NATO system ... therefore we propose the United States to establish a technical working group to make sure that this system will not be a threat - neither to (U.S.) F-35s nor the NATO systems,” Cavusoglu told a panel in the United States.