By Daren Butler

ISTANBUL (Reuters) – Turkey said on Tuesday a U.S. House of Representatives’ resolution condemning Ankara’s purchase of Russian defense systems and urging potential sanctions was unacceptably threatening.

Relations between the two NATO members have been strained on several fronts including Ankara’s plans to buy Russia’s S-400 air defense systems, the detention of U.S. consular staff in Turkey, and conflicting strategy over Syria and Iran.

The standoff threatens to bring U.S. sanctions, which would hurt Turkey’s already recession-hit economy, and raise questions over its role in the North Atlantic Treaty Organization.

The resolution, introduced in May and entitled “Expressing concern for the United States-Turkey alliance”, was agreed in the House on Monday.

It urges Turkey to cancel the S-400 purchase and calls for sanctions if it accepts their delivery, which may come as soon as July. That, the resolution said, would undermine the U.S.-led transatlantic defense alliance.

In response, Turkey’s Foreign Ministry said in a statement that its foreign policy and judicial system were being maligned by “unfair” and “unfounded” allegations in the resolution.

“It is unacceptable to take decisions which do not serve to increase mutual trust, to continue to keep the language of threats and sanctions on the agenda and to set various artificial deadlines,” it added.

PILOT PROGRAM WOUND DOWN

President Tayyip Erdogan’s government faces a balancing act in its ties with the West and Russia, with which it has close energy ties and is also cooperating in neighboring Syria.

The United States is also pressuring Turkey and other nations to isolate Iran, including blocking oil exports.

U.S. officials said on Monday the training of Turkish pilots on F-35 fighter jets had come to a faster-than-expected halt at an air base in Arizona, as Ankara’s involvement was wound down over the S-400 controversy.

The United States says Turkey’s acquisition of Russia’s S-400 air defenses poses a threat to Lockheed Martin Corp’s F-35 stealth fighters, which Turkey also plans to buy.

“We rarely see it in foreign affairs, but this is a black and white issue. There is no middle ground. Either Mr. Erdogan cancels the Russian deal, or he doesn’t,” Eliot Engel, chairman of the House Committee on Foreign Affairs, said on the House floor on Monday.

“There is no future for Turkey having both Russian weapons and American F-35s. There’s no third option.”

Regardless of the U.S. warnings, Turkey appeared to be moving ahead with the S-400 purchase. Erdogan said last week it was “out of the question” for Turkey to back away from its deal with Moscow.

(Reporting by Daren Butler; Editing by Jonathan Spicer and Andrew Cawthorne)