by Xinhua writers Zhu Dongyang, Liu Chen

WASHINGTON, July 30 (Xinhua) -- Turkey responded with a strongly worded statement to Washington's sanction threat over Ankara's detention of a U.S. pastor, escalating the tit-for-tat war of words between the two sides.

U.S. experts said that the current diplomatic spat between the two members of the North Atlantic Treaty Organization (NATO) may exacerbate their already tense relations.

TUG OF WAR

The 50-year-old pastor, Andrew Brunson, has been the latest irritant between the two nations. U.S. Secretary of State Mike Pompeo spoke with his Turkish counterpart Mevlut Cavusoglu twice last week, but failed to convince Turkey to send Brunson back.

U.S. Vice President Mike Pence said Thursday that the United States will slap significant sanctions on Turkey unless it releases Brunson "immediately."

President Donald Trump also said earlier that Brunson's detention was "a total disgrace," and urged Ankara to release him soon.

Turkey has not so far succumbed. Its National Security Council said on Monday that the threatening language of the United States is "unacceptable and disrespectful," and that Washington's stance on Turkey's defense industry projects will cause "irreparable damage" to the strategic partnership between the countries.

In response to Pence's sanction threat, Cavusoglu tweeted on Friday that Turkey "will never tolerate threats from anybody."

Turkish Vice President Fuat Oktay also joined the chorus, denouncing Washington's rhetoric as a "cheap threat."

Brunson was indicted for allegedly having links with the outlawed Kurdistan Workers' Party and Fethullah Gulen.

Ankara has accused Gulen, an exiled Turkish preacher now based in the United States, of masterminding the failed coup attempt in 2016 to topple President Recep Tayyip Erdogan.

A Turkish court ruled on Wednesday, agreeing to move Brunson from prison to house arrest over health concerns, but it still does not allow him to return to the United States.

Pompeo said later that the ruling is "long overdue" and "is not enough," urging the Turkish authorities "to resolve his case immediately in a transparent and fair manner."

The U.S. Senate introduced a bipartisan legislation on July 19 to restrict loans from international financial institutions to Turkey "until the Turkish government ends the unjust detention of U.S. citizens."

LONG LIST OF GRIEVANCES

Besides Brunson's case, there have already been too many issues between the two countries hampering a quick rapprochement, said Wayne White, former deputy director of the Near East and South Asia Office in the U.S. State Department's Bureau of Intelligence and Research.

Such issues, White said, include Erdogan's success in overcoming the coup and subsequent clinging to power, the detention of U.S. citizens in Turkey, as well as the row over Gulen, whom Turkey has repeatedly asked the United States to repatriate. Erdogan's criticism of U.S. military presence in Syria also added to the hostilities between Ankara and Washington.

Meanwhile, Turkey's purchase of Russian S-400 missile defense system has become a new flashpoint. The U.S. Congress has introduced another bill to prevent the transfer of U.S.-built F-35 fighter jets to Turkey in light of the Turkey-Russia deal.

The tense relationship between the United States and Turkey is reaching an "inflection point," Sloat said. "The United States has begun questioning whether Turkey is still a reliable ally."

Yet from Turkey's point of view, Washington has failed to take seriously its security challenges in Syria or comprehend the "trauma" of the coup attempt, experts said.

David Pollock, a scholar at the Washington Institute for Near East Policy, called the detention of Brunson "a form of hostage diplomacy."

"In other words, it's a deliberate attempt to set up some sort of false equivalence and potential for trade to free the American prisoners in exchange for Gulen," he said.

Regarding Washington's future response, Pollock said sanctions are "definitely on the table," especially because Trump has publicly warned about them and the Congress has been very interested in following them up.

"This is an issue on which the U.S. government ... is likely to hold very firm, that this is just not acceptable, and we can't play this game with Turkey," he added.

White predicted that Trump, along with Congressional Republicans, will be forced to impose sanctions by the party's powerful Christian right.

FURTHER APART

Pollack said the current spat won't wither the U.S.-Turkish partnership. On issues ranging from Syria and arms purchase, the two countries are "still engaged in trying to maintain the partnership," he added.

"If you look at the most significant issues, the two countries are trying to find middle ground."

Other experts, however, expect the sanctions to further drift Turkey away from the West.

"Sanctions are unlikely to free Brunson, but could well drive Turkey to increasingly 'go its own way,' as Erdogan has threatened," White said.

"In fact, if the stalled F-35 delivery becomes even less likely, Erdogan could be tempted to approach Moscow for a deal, securing advanced Russian fighter aircraft instead."

He added that Turkey will continue to side with Russia and Iran, both of which are under U.S. sanctions. "It may only be a matter of time before the U.S., NATO, and Turkey effectively part ways."

(Matthew Rusling from Washington also contributed to the report.)