Soren Sondergaard from the socialist Red-Green Alliance told Danish public television TV2 that Ankara had accused him of "promoting a terrorist organisation".

"It is absolutely correct that I have visited those who fought against the Islamic State, the Syrian Democratic Forces (SDF)," he said, adding he was "proud of it".

The SDF, the Kurdish administration's de facto army, spearheaded the fight against the Islamic State group in Syria, driving it from its last stronghold in the country in 2019 with US backing.

Sondergaard said the Organization for Security and Cooperation in Europe (OSCE) had lodged a formal complaint with Ankara.

"This casts a shadow over the Turkish election, they are already demonstrating they want to control it," he said.

Turkey is holding one of the most important elections of its post-Ottoman history on May 14.

Secular leader Kemal Kilicdaroglu, 74, and his multi-faceted alliance are posing the toughest election challenge to President Recep Tayyip Erdogan since his Islamic-rooted party first swept to power in 2002.

Sondergaard said it was "a misconception" that election observers should be objective.

"No parliamentarian is objective. We are not civil servants. It is precisely by way of having different points of view participating that it becomes objective," he said.

"A country can't pick and choose the parliamentarians that are serving as observers," he said.