The European Union could face a wave of returning battle-hardened Islamic State fighters from Syria unless it gets much tougher with Turkey, including breaking off any accession or trade talks, a senior Kurdish leader told Reuters.

President Donald Trump's announcement in early October that he was pulling U.S. forces from northeast Syria paved the way for a Turkish offensive against the Syrian Kurdish YPG militia who had been at the forefront of fighting against Islamic State.

Trump's move surprised both Britain and France and was cast as a betrayal by the Kurds, who lost thousands of fighters in the battle against Islamic State jihadists in the deadly crucible of Syria's 8-1/2 year war.

Ilham Ahmed, a Kurdish political leader and co-chair of the Syrian Democratic Council (SDC) executive, said in an interview that the EU should get tough with Turkey or it would soon face a wave of Islamic State militants arriving in Europe.

"The threat is very big due to the arbitrary way in which the United States has withdrawn. This has allowed many (Islamic State) members to escape and they will make their way back to their countries to continue their terrorist activities."

"This poses a big threat to Britain and to Europe in general," Ahmed added.

Islamic State (ISIS) once boasted a "caliphate" across swathes of Syria and Iraq and claimed deadly attacks across the world, though it is now in disarray, landless and leaderless after the killing of Abu Bakr al-Baghdadi during a U.S. special forces raid in northwestern Syria last month.

But in the tumult created by the U.S. withdrawal and the Turkish offensive, Ahmed said, Islamic State fighters could escape and travel over porous borders to Europe.

She called on Europe to send 2,000 troops to secure the Syrian-Turkish border and prevent fighters crossing, and to cease all arms sales to Turkey. "Our people are being killed by European weapons," she said.

Turkey says it will ensure that any Islamic State detainees in territory it has captured will remain in detention.

CHEMICAL WEAPONS ACCUSATION

Turkey views the Kurdish YPG militia as a terrorist organisation because of its links to Kurdish militants in southeastern Turkey. The SDC is the political wing of the Syrian Democratic Forces, of which the YPG is the main component.

The Kurds are mainly Sunni Muslims who speak a language related to Farsi and live in a mountainous region straddling the borders of Armenia, Iraq, Iran, Syria and Turkey. They have never obtained a permanent nation state.

Ahmed also said the West should investigate Turkey's alleged use of chemical weapons against the Kurds. She called on Europe to ensure Ankara was held responsible for what she said were Turkish war crimes during its offensive.

"EU-candidate Turkey is not the same Turkey you think you know - it is now a radical Islamist state and you, Europe, should understand that," Ahmed said, adding that the EU should cut off accession talks with Turkey and scrap any trade deals.

"Turkey needs to be afraid and it is not right now," Ahmed said, adding that top level IS militants had found refuge in Turkish-controlled areas of northern Syria.

A senior U.S. State Department official said last week that about 99 percent of some 10,000 suspected IS militants captured and jailed in YPG-controlled areas of northern Syria since the demise of their caliphate remain incarcerated, "and we're quite confident that that's going to remain that way".

Turkey has said it would never use chemical weapons and that it has done its utmost to minimise civilian casualties or damage to any religious or historic buildings during the offensive.

When asked about Trump's comment - in response to accusations of betrayal - that the Kurds had not fought alongside Allied forces on the beaches of Normandy in 1944 against Nazi Germany, Ahmed said Turkey had not been on the side of the Americans in World War Two.

"At the time of Normandy there was no Kurdish state or Kurdish entity to fight on behalf of the Americans - and Kurds were the victims in that war while the Turks were not with the Americans at the time. So I don't know why Trump would say what he said," she said.