The US maintains that the group is still linked to al Qaeda, however.
IS seized vast areas of Iraq and Syria in 2014.
Erdogan also said that Russia has agreed to provide air support to the operation, however, there has been no official comment by the Russian Defense Ministry yet supporting the claim.
Calling the operation a "new step" to establish security in Idlib, he promised Turkey would not desert civilians there.
"Today in Idlib there is a serious operation underway, and it will continue", Erdogan said during an address to his party in Afyonkarahisar.
Many Syrians had fled to Idlib from the neighbouring Aleppo province, which was rocked by heavy fighting previous year, and Turkey meant to support them, Erdoğan said.
The spokesman noted that the establishment of a de-escalation zone in rebel-held Idlib Province would start in the coming days.
By joining the Idlib mission, Turkey is "de facto agreeing to the transition of power for Assad", Talha Kose, an analyst with the Ankara-based Foundation for Political and Social Research, said at a conference on Syria in Istanbul on Thursday.
Bombardier slapped with second tariff
Prime Minister Justin Trudeau and May are appealing directly to U.S. Ross in a press release. "We will stand up and we will fight".
The Hurriyet daily said ultimately Turkey would ensure security in Idlib city and Russian Federation in the surrounding area.
The Turkish army has always been reinforcing its military deployment on the Syrian border in case of a threat to its territory. It said the operation was yet to formally begin.
Fighting broke out on the border of Turkey and the Syrian province of Idlib on Sunday ahead of a Turkish advance into the region.
"When you enter a boxing match, you don't count how many punches you will throw", Erdogan said. He spoke on condition of anonymity because he was not authorized to speak to reporters.
TRT World's Sara Firth, who is in the southern Turkish province of Hatay on the Mediterranean coast, explains the latest efforts to build a safe zone in northern Syria. At the same time, the Turkish military could launch an operation within Idlib.
On Saturday, the USA said it backed Turkish plans to intervene in northern Syria.
Afer those talks Putin declared the right conditions now existed to end the over six-year civil war that has killed an estimated 330,000 people since 2011.
The intermittent clashes were reported near the town of Kafr-Lusin on the border in the northern countryside of Idlib province in northwestern Syria, Xinhua cited local media as reporting. Turkey, however, has backed rebels seeking Assad's ouster.