On the profile page for the PKK -- listed as a terrorist organization by the U.S. -- found on the National Counterterrorism Center's (NCTC) website, the Democratic Union Party (PYD) is mentioned as a cause for a "heightened threat to Turkey and increased tensions along the border".

The PYD "has increased its presence in northern Syria along the border with Turkey by establishing control in Kurdish areas," the document says.

In the 2015 edition of the Counterterrorism Calendar issued by the NCTC, this particular reference to the group has been edited out, while careful readers will pick it up in the previous edition of 2014, as well as on the dedicated webpage still online as of Sunday, Feb. 21.

The dynamics of the links between Turkey, the U.S. and the PYD has caused a strain in the ties between Ankara and Washington.

U.S. officials have been adamant in recent weeks in refusing to recognize the PYD -- or its armed wing YPG -- as a terrorist organization, instead calling the YPG a "reliable partner" in the fight against Daesh, supporting the group with weapons and training.

Ankara considers all three -- the PYD, YPG and the PKK -- as terrorist outfits, and has frequently pointed to the link between these groups active in Turkey and Syria.

Turkish Prime Minister Ahmet Davutoglu has dubbed the YPG "a pawn of the Syrian regime" over the Feb. 17 Ankara attack that left 28 people dead and 81 injured. Turkish officials have revealed that the attack was carried out jointly by a Syrian-national YPG member and PKK terrorists based in Turkey.

A day before the attack, Syria's envoy to the UN said the PYD enjoys the support of not only the U.S. but also the Assad regime.

"These Syrian Kurds supported by the American administration are also supported by the Syrian government, just for your kind information", Bashar Ja'afari told reporters.

Ja'afari's words marked one of the Syrian regime's first direct acknowledgements of its cooperation with the PYD. However, it was not unexpected.

On Oct. 23, Russian President Vladimir Putin said that the Assad regime and PYD need to unite forces, calling on the group to come over to the regime's side in the Syrian conflict.

The war, which has left the country in ruins after five years of fighting, has resulted in the deaths of more than 250,000 people and displaced 10 million, according to UN figures.

It has also made Turkey the country hosting the biggest number of refugees in the world, with 2.62 million Syrian war survivors and 26 different camps.

Local sources near the Turkey-Syria border have told Anadolu Agency that Russian airstrikes have been facilitating the PYD's advance along the border.

The Turkish military shelled PYD/YPG positions in northern Syria for a week beginning Feb. 13 in response to artillery fire from PYD forces based around Aleppo's Azaz town, located just 6 kilometers (4 miles) from the Turkish border.

Announcing the military measure on Feb. 13, Prime Minister Davutoglu demanded the YPG withdraw from Azaz and the nearby Menagh military air base and warned the group against using either as a base to attack Turkey or Syrian opposition forces.

In an apparent reference to Russia, he said “forces using the PYD” had sought to cause a “massive refugee influx” towards Turkey. He added that Ankara was ready to take further measures to defend its borders.

Meanwhile in Washington, U.S. State Department spokesman John Kirby urged Turkey to halt its cross-border artillery fire and called on the YPG and its affiliated forces "not to take advantage of a confused situation by seizing new territory".

The link to the NCTC's PKK (KGK) profile can be found at . The agency defines the PKK as a precursor to Kongra-Gel (KGK), an umbrella organization that includes other domestic militant groups.

Anadolu Agency