HTS is not party to a deal brokered by Russia, Turkey and Iran for the safe zone in the province, one of four such "de-escalation" zones across Syria.
Rebels said they were preparing to start the operation soon in Idlib province and surrounding areas, which represent Syria's most populous rebel-held area, and residents reported Turkish authorities removing sections of a border wall.
"Today there's a serious operation in Idlib and it will continue", Erdogan said in a televised speech.
"Now this step has been taken, and it is underway", he said, adding that Turkish forces were not yet involved and that it was a rebel operation so far.
"For now the Free Syrian Army is carrying out the operation there", Erdogan said.
The president also said that Russian Federation and Turkey are acting jointly regarding the protection of people in Idlib, while adding that Turkey will provide security for Idlib's interiors and Russian Federation will protect the outer and border areas.
Speaking at AK party's camp in Afyonkarahisar, President Erdogan said, "We are now striving to move forward with a new step towards ensuring safety of Idlib in field of Euphrates shield operation".
Saying that the Turkish Armed Forces (TSK) are backing the operation from inside the Turkish border, Erdoğan noted that Russian Federation is providing aerial support.More news: Mourinho Does Not Want to be Complacent with MU's Brilliant Start
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Earlier on Saturday, Turkey continued to build up its number of troops on the southern border with Syria, close to the earmarked de-escalation zone near the city of Idlib.
The joint mission aims to promote an end to conflict in the region, fight al-Qaeda-linked Jabhat Fateh al Sham and Daesh.
The Hamza Brigade, also part of Euphrates Shield, posted video online of what it said was a convoy of its forces heading for Idlib.
Tahrir al-Sham is spearheaded by the former Nusra Front, which was al Qaeda's Syrian branch until past year, when it changed its name and broke formal allegiance to the global movement founded by Osama bin Laden.
Since early this year, it has battled other rebel groups in Idlib and other parts of Syria, while the government has mostly focused on the war against Islamic State in the east.
Many Syrians had fled to Idlib from the neighbouring Aleppo province, which was rocked by heavy fighting past year, and Turkey meant to support them, Erdoğan said.
Two waves of air strikes on Idlib killed at least 13 civilians over the past 24 hours, a Britain-based monitoring group said on Saturday.
SOHR in the last week has repeatedly accused Syria and Russian Federation of carrying out deadly air strikes in Idlib province with heavy civilian losses.