Some of the Kurdish forces had helped the USA to fight the Islamic State group.
The US has opposed Turkish attacks on the Syrian Kurds for years, and in recent days suggested Turkey should hold off on such operations, instead focusing on ISIS.
"ISIS is still present" and a lethal force, the State Department official said using an acronym for the group.
Turkey's government had named the offensive "Operation Olive Branch", according to a military statement, which asserted that the Turkish military action was aimed both at Kurdish militias as well as the Islamic State. Turkish reports earlier this week suggested that Russian observers vacated Afrin in anticipation of the upcoming offensive, but Russian Foreign Minister Sergey Lavrov denied this on Friday, according to Russia's official RIA Novosti outlet. Tensions between Turkey and the U.S. are likely to escalate.
Although Canikli said no Turkish troops have gone into Afrin, Turkish newspapers said 20 buses carrying Free Syrian Army rebels crossed on Friday from Turkey into a Turkish-controlled part of northern Syria, on Afrin's eastern flank.
"The Manbij operation will follow", he said during a televised speech in the Aegean province of Kutahya.
Turkey's military plans seem to have been accelerated by an announcement from the U.S. that it will help the Syrian Democratic Forces (SDF) - an alliance against IS of which the YPG is a member - build a new "border security force" to prevent the return of IS.
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But the YPG has been the key ally of Turkey's fellow North Atlantic Treaty Organisation member the United States in the fight against IS jihadists, playing a key role in pushing the extremists out of their Syrian strongholds.
What is happening in Afrin?
In a stark warning to Turkey, Syria said on Thursday said its air defense would shoot down any Turkish jets that carry out attacks within Syria. The staff also clarified that it would respect territorial integrity of Syria. "But that does not mean we will accept being treated as sacrificial animals just because a couple of American generals want to embark on an adventure in the Middle East", Ilnur Cevik, an Erdogan presidential adviser, wrote in the Turkish daily Sabah. But Turkey also fears any gains in strength for the Syrian Kurds, whose territory runs along some of Turkey's south border.
A spokesperson for the YPG told Reuters a number of people had been injured in the strikes on Afrin city and villages around it, but it is not yet clear how many.
Canikli also said Turkey had detailed information about the YPG's military capabilities, adding that Turkey had developed sophisticated weapons since its last incursion into Syria in 2016 that were able to counter them.
They said the "sudden and unjustified" attacks on Afrin "threatens to breath new life" in Isis which the militants are now fighting.
The YPG has held Afrin since 2012 when Syrian government forces withdrew and essentially handed over large swaths of the northern part of the country to the Syrian Kurds.
Any ground operation would entail considerable military and political risk for Ankara.