Mr Trump issued a brief statement overnight saying that America “does not endorse this attack”.
The Independent has that in more detail:
The comment was made in a statement released by the White House, in which he claimed Turkey is committed to ensuring the safety of civilians, in spite of concerns that the country will target Kurdish fighters who helped fight against the Isis insurgency there.
"This morning, Turkey, a NATO member, invaded Syria. The United States does not endorse this attack and has made it clear to Turkey that this operation is a bad idea," the statement says. "There are no American soldiers in the area. From the first day I entered the political arena, I made it clear that I did not want to fight these endless, senseless wars—especially those that don’t benefit the United States."
He continues: "Turkey has committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place—and we will hold them to this commitment. In addition, Turkey is now responsible for ensuring all ISIS fighters being held captive remain in prison and that ISIS does not reconstitute in any way, shape, or form. We expect Turkey to abide by all of its commitments, and we continue to monitor the situation closely."
Pompeo also went on record to say that the US did not approve the latest Turkish offensive beforehand. As the BBC summarized:
In an interview with US broadcaster PBS, Mr Pompeo defended Mr Trump's surprise decision to pull back US forces, adding that Turkey has a "legitimate security concern" and "a terrorist threat to their south".
He said reports the US had allowed Turkey to launch the offensive were "just false".
"The United States didn't give Turkey a green light," he said.
The BBC also summarized this bit from Trump
The president later told a press conference the Turks and Kurds "have been fighting each other for centuries", and said that Kurdish fighters "didn't help us in the Second World War, they didn't help us with [the D-Day landings in] Normandy".
But "later" is relative to an earlier post of Trump, not to Pompeo's statement. So that appears to be the last thing that Trump said publicly (on the matter) before the current Turkish offensive began. In the same statement Trump also compared the Turkey-Kurds situation with Israel vs Palestinians.