Jets conducted operations up to 30 km (18 miles) into Syria, and a Reuters witness saw shells exploding just outside the town of Tel Abyad. "109 terrorists have been killed so far", Erdogan said in a speech to members of his AK Party in Ankara.
In sharp-edged remarks aimed at Turkey, Juncker pointed out that the European Union is contributing €6bn (£5.4bn) to Turkey to help support the 3.6 million Syrian refugees in the country, adding that Europe would not contribute to the creation of a "safe zone" in northern Syria.
Erdogan sought to assuage those concerns, saying that militants from the jihadist group would not be allowed to rebuild a presence in the region. "If the EU and European countries don't want to work with Turkey then it will be their problem as well".
Egypt and Saudi Arabia both came out in opposition to the Turkish advance.
American intelligence officials told NBC that they fear "a replay of what happened in Iraq between 2010 and 2013, when the core group who founded ISIS were released or escaped from detention after U.S. forces left the country".
The decision has been widely criticized as an abandonment of Syrian Kurds.
Stoltenberg told a news conference in Rome that while Turkey, a North Atlantic Treaty Organisation ally, "has legitimate security concerns, ... it is important to avoid actions that may further destabilise the region, escalate tensions and cause more human suffering".
The SDF have served as ground troops for the US-led campaign against Islamic State (IS, formerly ISIS) terrorists since 2015.
Turkey has the largest refugee population in the world, and the presence of Syrians in the country has increasingly led to animosity from Turks who blame them for crime, unemployment and the dilution of Turkish culture.More news: Coco Gauff reaches maiden WTA final at Linz Open, meets Jelena Ostapenko
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The group claims to espouse an ideology called "democratic confederalism", based on the ideology of PKK founder and leader Abdullah Ocalan, who has been in prison in Turkey since 1999.
Trump insisted that he did not endorse the Turkish invasion in any way, calling it a "bad idea", and yet again promised to "wipe out" Turkey's economy if Ankara fails to protect civilians and religious minorities, or fails to ensure no humanitarian crisis takes place.
US senators on October 9 announced a bipartisan bill was being drafted to sanction the heads of the Turkish political leadership, including President Recep Tayyip Erdogan, and the country's military and energy industries.
Sanctions would also be imposed on Turkey for its purchase of the Russian S-400 anti-air missile system.
When asked Wednesday about the threat of ISIS prisoners escaping, Trump claimed that some of the most unsafe ISIS prisoners had been moved, "putting them in other areas where it's secure".
After Turkey launched its operation, Britain, France and Germany called for an emergency UN Security Council meeting to be held on Thursday.
On Friday, the Pentagon said its base near the northern Syrian town of Kobane - an area known by Turkey to have United States forces present - had seen shell fire from Turkish positions.
"As Turkish offensive in Syria begins, the IRC is deeply concerned about the lives and livelihoods of the two million civilians in northeast Syria who have already survived ISIS brutality and multiple displacements", the International Rescue Committee said in a statement Thursday.
Israeli Prime Minister Benjamin Netanyahu condemned Turkey's military incursion and cautioned about the possibility of ethnic cleansing.