Turkish security forces arrested two foreign nationals in the capital Ankara for their suspected links to Daesh, a security source said Sunday.

The suspects were arrested in a joint operation by counterterrorism units of the provincial police department and the Turkish National Intelligence Organization (MIT), said the source, who asked not to be named due to restrictions on speaking to the media.

Police also seized a number of documents and digital materials in the operation.

Footage of the terrorist attack in Vienna was also found in the digital materials seized in the operation.

At least four people died and 22 others were wounded in the terrorist attack in the Austrian capital on the night of Nov. 2.

Separately, Turkish authorities on Saturday identified two Daesh terrorists, who were arrested on Friday in an operation in Turkey, a security source said.

One of the suspects, identified by the initials H.E., was Daesh's so-called Raqqah emir between 2014-2018 in Syria and carried out illegal activities in Turkey to keep the terrorist group alive, said the source, who asked not to be named because of restrictions on speaking to the media.

The other one, Y.S., was identified by authorities as an executioner for the terrorist group.

Meanwhile, it was also reported that the PKK terrorist group’s Syrian branch, the YPG, on Friday set a ringleader of the Daesh terrorist organization in Syria free, according to local sources.

YPG terrorists released the so-called "commander of public relations bureau," Aid Abdulhamid al-Deyri, who was detained by the terrorist group two years ago, according to sources.

The terrorist was previously involved in several war crimes, sources said.

Previously, YPG terrorists released hundreds of Daesh members from camps in northeastern Syria.

Last month, the YPG announced it set 631 terrorists free.

Although Daesh has been largely defeated in Iraq and Syria, its presence still poses a threat, as individuals following its ideology encourage others to carry out violence. European analysts have also warned against attacks by Daesh, as attacks by isolated individuals not under the watch of intelligence services have become more common.

Turkey recognized Daesh as a terrorist group in 2013, and since then, the country has been attacked numerous times, including 10 suicide bombings, seven bombings and four armed assaults, which have killed 315 people and injured hundreds more.

In response, Turkey launched military and police operations both inside and outside the country, capturing top Daesh members in counterterrorism efforts at home and in Syria.

Turkish intelligence played a key role in the death of Daesh leader Abu Bakr al-Baghdadi by detaining and extraditing one of his aides to Iraq, who provided U.S. authorities with critical information for locating him.