Turkey announced the completion of its half-year Euphrates Shield operation in northern Syria against both jihadists.
Yildirim appeared to contradict comments Saturday from the Kurdish-Arab alliance battling the jihadists that the push into the northern Syrian city would "begin in a few days".
"The long-planned Raqa operation began late on June 2," after the US informed Turkey of the operation, Yildirim said, without providing further details.
He was referring to the latest thrust in a drive by US-backed Syrian Democratic Forces (SDF) to close in on Raqa that began in November.
The SDF is a Kurdish-Syrian Arab alliance including fighters from the Kurdish Peoples' Protection Units (YPG), which Ankara views as a "terror group" linked to Kurdistan Workers' Party (PKK) separatists inside Turkey.
SDF seized a town and dam to the west of Raqa this weekend.
"We will begin in a few days," said Jihan Sheikh Ahmed, spokeswoman for the SDF's "Wrath of the Euphrates" operation to capture Raqa, speaking Saturday.
The Raqa operation has been a deep source of tension between Washington and Ankara because of US support to YPG, which has included air cover, assistance from special forces on the ground and weaponry.
The US said last week it began sending arms to Kurdish fighters, which Turkey said was "extremely dangerous" and urged Washington to reverse the decision.
Washington views the YPG as the most effective fighting force against IS.
But Yildirim said its NATO ally had downplayed its support for YPG, referring to the relationship as "tactical cooperation", according to comments reported by Anadolu Sunday.
In April, Turkey announced the completion of its half-year Euphrates Shield operation in northern Syria against both jihadists and Kurdish militia.
But during his briefing with journalists on Saturday evening, according to Anadolu, Yildirim suggested Turkey would not shy away from further action, without giving further detail.
"Whether inside or outside the country, we will not hesitate or hold back from taking the necessary steps for the security of our country and people," Anadolu quoted him as saying.
"We've done this in the past and after this if it is required, we would do so again," Yildirim added.