“Turkey belongs to Europe and no one can change that. Thus, Turkey's and the EU's future are parallel and indexed to each other," Gunter Verheugen, who served as EU commissioner for enlargement from 1999 to 2004, told Anadolu Agency in an interview.

Addressing recent incidents of terrorist PKK supporters demonstrating in European cities using terrorist images with impunity, he said: "Believe me, it also shocks me how the PKK could freely spread propaganda, mount protests, put up posters, and hang banners in Brussels and other European cities while the police disregards it and doesn’t intervene."

For more than 30 years, the PKK has waged a terror campaign against Turkey leading to the deaths of more than 40,000 people -- security forces and civilians alike -- including more than 1,200 since July 2015 alone.

Verheugen criticized a selective approach to terror groups, saying: "All terrorist groups must face the same treatment and position without any exceptions in the fight against terrorism."

He also warned that Turkish-EU relations may further "deteriorate" in the near term, citing worsened relations with Britain seen in the Brexit process.

"It’s clear that the Brexit hasn’t brought any sort of benefit but from a long-term perspective, I must say I’m optimistic [for Turkey]. In my opinion, if we’re discussing the EU’s future and evaluating its situation under a united EU, then the only situation we can see is Turkey's EU membership,” he said.

"We can discuss two scenarios. Under one, the EU is forced to accept Turkey after a certain process. Under another, [the EU] analyzes and understands the situation thoroughly and makes a decision regarding Turkey at once without being forced to do so. I hope the second happens. Turkey belongs to Europe and no one can change that," he said.

Verheugen is in Turkey this week for a summit in Kocaeli, northwestern Turkey titled “International Symposium on July 15 and Coups D'etat - Global Impacts, Media and Democracy.”

The Fetullah Terrorist Organization (FETO) and its U.S.-based leader Fetullah Gulen orchestrated the defeated coup attempt of July 15, 2016 in Turkey which left 250 people martyred and nearly 2,200 injured.

Ankara accuses FETO of being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police, and judiciary.

The summit, which ends Saturday, is hosted by the Kocaeli Greater Municipality, and Anadolu Agency is its global communication partner.

In 1987 Turkey applied for membership in the European Economic Community, a precursor to the EU. It became eligible for EU membership in 1997, and accession talks began in 2005.

To gain membership, Turkey has to successfully conclude negotiations with the EU in 35 policy chapters that involve reforms and the adoption of European standards.