EU chief Jean-Claude Juncker spoke as Turkish forces launched an assault on Kurdish positions, with air strikes and explosions reported near the border.

Turkish President Recep Tayyip Erdogan announced the start of the attack, which is aimed at curbing the Kurdish-led Syrian Democratic Forces (SDF).

The SDF was previously allied to the US, which used it to crush the Islamic State group.

Juncker told the European Parliament he recognised Turkey had "security concerns" along the border. But he warned the military action would not lead to a "good result", saying a political solution was the only way to end the Syrian conflict.

"I call on Turkey as well as the other actors to act with restraint and to stop operations already, as we are speaking, under way," Juncker said.

"I have to say if the Turkish plan involves the creation of a so-called safe zone, don't expect the European Union to pay for any of it."

- 'Fertile ground' for IS -

Ankara regards the Kurdish fighters as terrorists linked to insurgents inside Turkey. It wants to invade to create a "safe zone" on the Syrian side of the border where it could send back some of the 3.6 million refugees it hosts from Syria's eight-year civil war.

The EU's top foreign policy official, Federica Mogherini, issued a statement echoing Juncker's declaration and warning that "unilateral action on Turkey's part threatens" concerted action by the West and Turkey and other countries to defeat the Islamic State (IS) group, also known in Arabic as Daesh.

Turkish military action, she said, risked "protracted instability in northeast Syria, providing fertile ground for the resurgence of Daesh".

Keeping captured IS fighters in Syria secure is "imperative in order to prevent them from joining the ranks of terrorist groups," she added.

Turkey on Wednesday called on Europe to take back their citizens among the ex-fighters' ranks, though it pledged it would not allow them to be released.

Like Juncker, Mogherini warned that any supposed "safe zone" Turkey created would be unlikely to meet the criteria needed for Syrian refugees to be placed there in "safe, voluntary and dignified" conditions.

Any attempt at demographic change would be unacceptable. The EU will not provide stabilisation or development assistance in areas where the rights of local populations are ignored," she said.

- NATO member -

Ankara agreed a six-billion-euro ($6.6-billion) deal with Brussels to stem the influx of migrants to the EU in 2016, but relations soured after Erdogan unleashed a major crackdown in the wake of a failed coup to overthrow him in the same year.

NATO member Turkey is technically a candidate country to join the European Union but accession talks are effectively frozen and ties between the Ankara and the bloc have limped along for years.

NATO chief Jens Stoltenberg issued his own statement urging "restraint" and pointing out the risk of worsening instability in Syria, though he did acknowledge Turkey had "legitimate security concerns" in the area.