The US move to suspend visa services in Turkey was motivated both by Washington's concern over Ankara's Syria policy, and by last week's arrest of a US embassy employee who allegedly had information on American involvement in the 2016 failed coup attempt, a senior Turkish official told Al Jazeera.

The US mission in Turkey announced on Sunday that it had stopped all non-immigrant visa services amid concerns over "the security of US mission and personnel". Ankara reiterated reciprocally hours later, using similar language.

The development is unprecedented between the two NATO allies and represents a major fallout in bilateral relations.

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Turkish authorities last week detained Metin Topuz, a Turkish citizen working for the US consulate in Istanbul. He was accused of having links to the organisation of Fethullah Gulen, an exiled religious leader and businessman based in the US and wanted in Turkey. Ankara accuses Gulen of masterminding the July 15 coup attempt that killed more than 300 people.

Turkish President Recep Tayyip Erdogan has repeatedly called on Washington to extradite Gulen since the coup attempt, but the US has refused.

Another US consulate employee was called in for questioning by the Istanbul prosecutor's office on Monday as his son and wife were taken into custody in Amasya province. Turkish authorities say that both Metin Topuz and the second unnamed employee have no diplomatic immunity or title.

Turkey summoned the US embassy undersecretary on Monday and called for the de-escalation of tensions and an end to the suspension of visa services.

Trump's appeal

Several US citizens were among the thousands arrested in Turkey after the coup attempt, including American pastor Andrew Brunson. President Donald Trump unsuccessfully appealed to Turkey for his release, along with a letter signed by 78 members of Congress.

Erdogan said last month that Ankara would release the pastor once Gulen was extradited to Turkey.

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The Turkish government says that the recent wave of detentions and purges in the public sector are aimed at removing Gulen supporters from state institutions and other parts of society.

Local and international rights groups, as well as many of Turkey's European allies, say the arrests and purges are arbitrary, accusing the government of using the coup attempt as a pretext to silence opposition in the country.

Burhan Kuzu, an adviser to Erdogan and a senior MP of the ruling Justice and Development Party, told Al Jazeera that the US was disturbed by the detention of Topuz, as he had information about US involvement in the coup attempt.

"The US is irritated by this development as its [Washington's] role in the coup attempt might be revealed through this individual. The visa move came in order to pressure Turkey to give up this person," he said.

In a written statement late on Monday, John Bass, the US ambassador to Turkey, said his mission had been unable to learn the reasons for the arrest of Topuz or what evidence exists against the employee.

Bass added that he had not been allowed sufficient access to the employee's lawyer.

He said that the Turkish move "has raised questions about whether the goal of some officials is to disrupt the long-standing cooperation" between Washington and Ankara.



"At this time, we can't predict how long it will take to resolve this matter," the ambassador said.

The US and Turkey have also recently been at odds over Syria. Ankara, which backed the Syrian uprising, has negotiated multiple agreements with Russia and Iran, supporters of the Syrian government.

Turkish forces on Monday crossed into Syria's Idlib province to carry out "expeditionary activities aimed at preventing violence" in line with one of these agreements, which stipulated the creation of "de-escalation zones" in Syria.

The US and its allies, meanwhile, have been using Turkey's Incirlik airbase for operations against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

US support for Kurdish groups

Ankara has also been displeased with Washington's move to deliver arms to Kurdish groups that Turkey views as "terrorists". The Kurdish People's Protection Units (YPG), the main part of the Syrian Democratic Forces set up to fight against ISIL, have received arms shipments from the US.

Ankara views the YPG as an extension of the Kurdistan Workers' Party (PKK), which has been waging a war against Turkey for more than 35 years.

Kuzu criticised the US for supporting "Kurdish terrorist groups" instead of Turkey, adding that Washington had problems with Turkey's cooperation with Iran and Russia in Syria. "Washington, supposedly our ally, aims to create a Kurdish corridor in the east of Turkey by supporting the YPG and PKK," he said, noting that Turkey's Euphrates Shield operation was aimed at preventing the emergence of this corridor.

Also feeding the escalation, Turkish officials have criticised the US for what they say is its support for a non-binding referendum for an independent Kurdish state in Iraq, and the issuance of arrest warrants for a group of Erdogan's bodyguards after a brawl during his visit to Washington, DC, in May.

Follow Umut Uras on Twitter: @Um_Uras