(Adds Pompeo-Cavusoglu meeting, meeting with lawmakers)
By Doina Chiacu and David Brunnstrom
WASHINGTON, April 3 (Reuters) - U.S. Vice President Mike Pence warned Turkey on Wednesday against buying a Russian S-400 anti-missile system, keeping up pressure on the NATO ally to abandon a purchase that Washington views as 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 an address during a meeting of NATO ministers in Washington.
The United States and Turkey remain at loggerheads over Ankara'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 Corp.
Turkey struck back promptly, with Turkish Vice President Fuat Oktay issuing his own admonition 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 to Turkey of equipment related to the stealthy aircraft.
"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.
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 Washington.
Ankara says it needs the S-400s, to be delivered in July, to defend itself, as Turkey faces threats from Kurdish and Islamist militants at home and conflicts in neighboring Syria and Iraq.
The United States hopes to find a way out of the dispute by convincing Turkey to purchase the U.S.-made Patriot air defense system, made by Raytheon. Turkey is also looking into a French-Italian system based on the SAMP-T missile systems.
Secretary of State Mike Pompeo discussed his concerns about the S-400 surface-to-air missile system with Cavusoglu in a meeting on Wednesday, the State Department said.
The leaders of the U.S. House of Representatives Foreign Affairs Committee, who can block military sales to Turkey, also told him that the S-400 purchase could jeopardize Turkey's future in NATO and risked sanctions as well as Turkey's participation in the F-35 program.
NATO Secretary-General Jens Stoltenberg, in Washington for the 70th anniversary of the Western alliance and a meeting of its foreign ministers, told Reuters in an interview he had spoken several times with Turkish President Tayyip Erdogan about the S-400 issue and had discussed it in a meeting with Trump on Tuesday.
Asked about Turkey's working-group proposal, Stoltenberg said the S-400 issue had been considered many times at different NATO ministerial meetings. While it was not formally on the agenda in Washington, he expected it to be addressed.
"I expect that on the margins of our foreign ministerial meeting here in Washington this week, the S-400 issue will be discussed again. Not at the formal agenda, but on the margins when we have the ministers sitting down and looking into whether there’s a way to find a way to find a compromise," Stoltenberg said.
In the meeting with his Turkish counterpart, Pompeo also called for the speedy resolution of cases involving detained U.S. citizens including Serkan Golge, the State Department said. (Reporting by Doina Chiacu and David Brunnstrom in Washington; Additional reporting by Sarah Dadouch in Istanbul and Patricia Zengerle in Washington; Editing by Grant McCool and James Dalgleish)