In a move that looks suspiciously like retaliation, Turkish police on Wednesday arrested a local employee of the US embassy in Istanbul and charged him with espionage and trying to overthrow the government, the Wall Street Journal reported, citing local media.

The arrest stemmed from the man’s alleged support for US-based Turkish Cleric Fettulah Gulen, a former ally of Turkish President Recep Tayyip Erdogan who escaped to the US nearly two decades ago after he and Erdogan had a falling out. The president has accused Gulen of orchestrating last June’s attempted coup from his home in Pennsylvania, and his administration has relentlessly purged suspected supporters from nearly every institution in Turkish society.

Unsurprisingly, the US government criticized the arrest, claiming the charges against the man were unfounded. Embassy officials told WSJ they were “deeply disturbed” by the arrest.

“We believe these allegations to be wholly without merit.” “Baseless, anonymous allegations against our employees undermine and devalue this longstanding partnership,” it said.

Specifically, the embassy employee is accused of having ties to a former prosecutor and four former police chiefs who led a corruption probe in 2013 that involved some of Erdogan’s allies. The government said cleric Fethullah Gulen’s movement pushed the investigation behind the scenes to try and undermine Erdogan, a claim Gulen denies. Gulen also denies involvement in last year’s coup attempt.

Turkey has been pressing the US to extradite Gulen, who is a wanted man in his home country, but the US has maintained that the evidence against Gulen isn’t strong enough to warrant extradition, straining the relationship between Trump and Erdogan. The two world leaders have praised one another, and Trump received Erdogan and his entourage at the White House in the spring.

As WSJ points out, Erdogan last month raised the possibility of releasing an American pastor, Andrew Brunson, who was detained almost a year ago in the wake of the coup attempt, in exchange for Gulen.

Brunson has been charged with terrorism-related activity and espionage for alleged ties to Gulen that the US has denounced as unfounded.

Of course, the diplomatic tug of war over Gulen is just one of several flashpoints in the US’s increasingly fraught relationship with Turkey, a fellow NATO member. Erdogan has been widely criticized for not doing enough to stop ISIS fighters from crossing Turkey’s porous southern border with Syria. In response, the president has repeatedly accused the US of supporting and arming ISIS, a claim the US has denied.

The relationship between the two countries was further strained last month when local prosecutors unveiled criminal charges against members of Erdogan’s security detail after his body guards viciously attacked a group of Kurdish protesters in Washington in the spring.

Just days after those charges were announced, Erdogan’s bodyguards brutally assaulted protesters who interrupted a speech Erdogan was delivering in New York City when he was in town for the UN General Assembly last month. The US has also criticized Erdogan’s increasingly paranoid crackdown on dissent, which has led to tens of thousands of arrests.