Mark Esper made the comments during a Pentagon briefing (Picture: PA).
The US Defense Secretary says there is "no indication" Turkey is willing to stop its offensive in north-east Syria.
Mark Esper said he is "greatly disappointed" by the actions of Turkey but admitted he is "not surprised".
The offensive comes after the withdrawal of US troops from Syria near the Turkey border.
US forces fought alongside Kurdish forces in the country against so-called Islamic State (IS).
Turkey, a NATO member, views Kurdish forces as terrorists.
"We oppose and are greatly disappointed by Turkey's decision to launch a unilateral military incursion into northern Syria," Mr Esper said during a Pentagon briefing.
"This operation puts our SDF [Syrian Democratic Forces] partners in harm's way, it risks the security of ISIS prison camps and will further destabilise the region."
Mr Esper also said he spoke to Turkey's defence minister yesterday to express America's "strong opposition".
The decision to withdraw American troops from the area has been widely criticised, even by loyal Republicans.
A former British SDF fighter labelled the move a "betrayal", while retired US military officer General David Petraeus raised concerns about America's "level of commitment" to its partners and allies.
However, Mr Esper told reporters: "We have not abandoned the Kurds, let me be clear about that.
"Nobody green-lighted this operation by Turkey, just the opposite.
"We pushed back very hard at all levels for the Turks not to commence this operation."
Mr Esper also defended the withdrawal, saying: "I will not place American service members in the middle of a long-standing conflict between the Turks and the Kurds.
"This is not why we are in Syria."
He also called on Turkey to "get back to the status quo" so the possibility of a safe zone along the border could be discussed.
The Turkish defence ministry said 342 "terrorists" have been "neutralised" since the offensive began on Wednesday.
At least six civilians are reported to have been killed in Turkey and seven more in Syria.
NATO Secretary General Jens Stoltenberg said he has "serious concerns" over the Turkish incursion - warning it could lead to "even more human suffering".
EU Council President Donald Tusk urged Turkey to halt the offensive before it triggers another "humanitarian catastrophe".
Meanwhile, the US also announced it is deploying 3,000 extra military personnel to Saudi Arabia.
The deployment includes two additional fighter squadrons, two Patriot batteries and one Terminal High Altitude Area Defense (THAAD) system.
It comes after an attack on a key Saudi oil facility last month which wiped out more than 5% of the world's oil production.
The US, among other nations including the UK, blamed Iran for the attack who denied responsibility.