The European Union on Saturday condemned as "extremely worrying" Turkey’s arrest of academics who signed a petition criticizing a military crackdown in the Kurdish-dominated southeast.

The European Union on Saturday condemned as "extremely worrying" Turkey's arrest of academics who signed a petition criticizing a military crackdown in the Kurdish-dominated southeast.

"The steps taken against the Turkish academics who signed a declaration regarding events in the southeast of Turkey are an extremely worrying development," an EU spokesperson on foreign affairs said in a statement.

"They are no longer detained, but the procedures against them are ongoing."

Turkish police on Friday detained at least 18 academics who signed a petition criticizing a military crackdown in the southeast, triggering new alarm about freedom of expression in the country.

They were arrested in raids targeting 21 academics accused of disseminating "terrorist propaganda" by signing a petition denouncing military operations against Kurdish rebels.

Fifteen academics and lecturers from the University of Kocaeli, near Istanbul, were initially detained, with another three academics from Uludag University in western Bursa province later held in their offices, Turkey's Dogan news agency said.

The academics were questioned for a day before being released, Dogan reported late Friday.

"While reaffirming our strongest condemnation of all forms of terrorist attacks, including by the PKK ... we restate that the fight against terrorism must fully respect obligations under international law, including human rights and humanitarian law", the EU spokesperson said.

"Freedom of expression must be upheld, in line with the Copenhagen political criteria; an intimidating climate goes against this.

"We expect Turkey ensure that its legislation is implemented in a manner which is in line with European standards enshrined in the European Convention for Human Rights and the case-law of the European Court of Human Rights."

Turkish prosecutors on Thursday launched a vast investigation into over 1,200 academics from 90 Turkish universities for engaging in "terrorist propaganda" and "inciting hatred and enmity" by signing the petition.

Entitled "We won't be a party to this crime", the petition urged Ankara to halt "its deliberate massacres and deportation of Kurdish and other peoples in the region", angering President Recep Tayyip Erdogan.

In a rare rebuke to Washington's NATO ally, the U.S. ambassador to Turkey has also expressed alarm over the investigations, with Western concern on freedom of expression already riding high due to the detention since November 26 of two prominent opposition journalists.

Turkey is waging an all-out offensive against the separatist Kurdistan Workers' Party (PKK), with military operations backed by curfews aimed at flushing out rebels from several southeastern urban centers.

But Kurdish activists say dozens of civilians have died as a result of excessive force and the operations have become the subject of huge controversy in Turkish society.