Ankara said it plans to target Syrian Kurdish YPG militia and ISIL militants during the offensive. "We're concerned about what this could mean for the potential for the resurgence of Daesh", he added, using the Arabic acronym for the Islamic State group.
Turkey's President Recep Tayyip Erdogan confirmed on Wednesday a bombardment on the region that sent thousands of civilians fleeing their homes. The airstrikes hit the elements in Ras al-Ain as well as towns in the rural areas of Derik and Qamasli districts.
But he rejected criticism from fellow Republicans over his decision to pull US troops out of northern Syria, and dismissed worries that captured Islamic State fighters might escape in the chaos of a Turkish attack.
Ankara has said it intends to create a "safe zone" in order to return millions of refugees to Syrian soil. But world powers fear the Turkish action could exacerbate the conflict, and run the risk of Daesh prisoners escaping from camps amid the chaos.
Erdogan sought to assuage those concerns, saying that militants from the jihadist group would not be allowed to rebuild a presence in the region.
In the statement the European Union "calls upon Turkey to cease the unilateral military action", adding that "renewed armed hostilities in the north-east will further undermine the stability of the whole region, exacerbate civilian suffering and provoke further displacements".
Egypt and Saudi Arabia both came out in opposition to the Turkish advance. "We, however, take action and that is the difference between us".
The attacks took place days after US President Donald Trump made the contradictory agreement to the Turkish military operation despite the US being an ally to the Kurdish led Syrian Democratic Forces (SDF).
Turkey was yet to unleash its full military might however, with the Syrian Democratic Forces - the autonomous Kurds' de facto army - holding off two incursion attempts.
"That's just false", he said, in an interview with PBS channel, but did not elaborate other than to say that Turkey has a "legitimate security concern". We could have given it to them, they could have had trials, they could have done whatever they wanted.
Kurdish leaders accuse the U.S. of stabbing them in the back.
After Mr Erdogan announced the offensive, Mr Trump called the operation "a bad idea" and later said he did not want to be involved in "endless, senseless wars". He expected Turkey to protect civilians and religious minorities and prevent a humanitarian crisis, he said.
U.S. Senator Lindsey Graham, a Republican who usually backs Trump, has been one of the most outspoken critics of the president's decision to withdraw U.S. troops from northeastern Syria.
Sanctions would also be imposed on Turkey for its purchase of the Russian S-400 anti-air missile system.
Australian Prime Minister Scott Morrison said Thursday he had been in contact with the Turkish and US governments overnight but admitted to being anxious about the situation.