The official said the group has spent the last several weeks preparing, including setting up safe houses, establishing informant networks and coordinating operations with Iraqi and Peshmerga units. It's the same strategy that Special Operations forces have used in previous deployments to combat zones.
Several Pentagon and military officials declined to discuss specifics of the so-called Expeditionary Targeting Force with CNN.
But Defense Secretary Ash Carter seemed to confirm in comments made at the Pentagon on Monday that the Special Operations forces had begun missions.
"The only thing I'll say is the (Expeditionary Targeting Force) is in position, it is having an effect and operating, and I expect it to be a very effective part of our acceleration campaign," he said during a press conference.
According to Carter, the force will conduct raids, seize places and people, and free ISIS-held hostages and prisoners.
Carter also told reporters that the force would cause ISIS "to fear that anywhere, anytime, it may be struck."
A U.S. official told CNN that Carter's statement reflects that Delta operations have begun.
CNN is not detailing any precise locations or operations. Based on several interviews with U.S. officials however, a growing role is rapidly emerging for Special Operations forces in Iraq and Syria.
CNN has learned that Delta Force plans to replicate the strategy that Special Operations forces used for years in Iraq and Afghanistan. The plan: Gather enough intelligence to stage raids on terror compounds and hideouts. Then from intelligence gathered at those sites, such as laptops and cellphones, forces will try to rapidly learn more about ISIS networks and quickly attack additional related targets.
It's a strategy that worked in May 2015, when Delta raided a compound in Syria, killing ISIS operative Abu Sayyaf and capturing his wife.
U.S. military officials have said material gathered at the raid and the interrogation of his wife provided extensive intelligence of ISIS networks that has been used in subsequent missions.
The Abu Sayyaf raid is the only known ground combat operation inside Syria for U.S. forces outside of a failed hostage rescue attempt. All other operations to kill ISIS operatives have been conducted by overhead drones. Putting forces on the ground is seen as a way to go more directly after key individuals, although it poses the risk of any ground combat operation.
Carter announced the initial details in December during congressional testimony, saying that the Pentagon was sending a "specialized expeditionary targeting force" to conduct "raids and intelligence gathering missions."
Carter also said at the time that "this force will also be in a position to conduct unilateral operations in Syria." For now, the force is in fact working only in Iraq, but is prepared to go into Syria if it gathers enough intelligence to warrant a mission.
Conducting targeted operations inside Syria is seen as potentially more risky because there would not be a local force on the ground for the U.S. troops to work with.
The ETF -- which numbers about 200 personnel -- has collected enough intelligence now about ISIS operations in Iraq in up to half a dozen locations that raids and field operations are ready inside Iraq.
These are described as "targeted" missions, in which the military is going after a specific individual or ISIS operation. Targeted missions required days, if not weeks, of continuous surveillance of the area to ensure civilians are not nearby. Depending on the situation, the President often will be asked to approved specific missions.
The personnel are largely made up of Delta Force, one of the U.S. military's so-called "Tier One" Special Operations units. As with the Naval Special Warfare Development Group -- publicly known as Seal Team 6 -- Delta operatives are highly trained to operate secretly in hostile environments.
There remains a raging debate inside the administration about whether to acknowledge the ETF operations once they begin. Gen. Joseph Votel, head of the the U.S. Special Operations Command has warned all his troops not to talk about it because of security concerns. But Pentagon officials acknowledged to CNN that there is pressure within the administration to tout the success of the effort -- if indeed the operations are successful.
While the ETF is not yet operating in Syria, a separate group of about 50 U.S. special forces have greatly expanded their initial operations there in recent weeks after the Pentagon announced they were going to Syria.
This group's mission is to accompany tribal, Arab and Kurdish forces in Syria as they try to regain territory from ISIS. Small U.S. teams now regularly leave their locations in northern Syria and go into the field. Several U.S. military officials confirmed that most recently U.S. troops went with local fighters to a location near the town of Al-Shaddadi in eastern Syria to help coordinate their operations and assist them in calling in airstrikes. ISIS has in recent days been driven from the area and U.S. officials see it as a victory for the policy because getting ISIS out of the town cuts a key route between Syria and Iraq.
But expanding Special Operations forces in Syria by sending the ETF remains complicated in addition to security concerns. If the teams capture operatives in Iraq, the plan calls for them to be turned over to the Iraqi government. If ISIS operatives are captured in Syria, the U.S. might try to turn them back over to their home countries, officials said. But clearly if there are Syrians and other nationalities fighting, that might make a turnover difficult and the U.S. is not planning to hold the people it captures beyond a short period of time.
And in another complication, so far it is not clear to what extent the U.S. would inform Russia of any ground operations to ensure U.S. troops are not inadvertently bombed in Russian airstrikes. Currently the Russians have only been given the broad indications, and not specifics, of where Special Operations force are based in northern Syria, U.S. officials said.
Carter also said Monday that "momentum is now on our side" in the fight against ISIS.
He told reporters that he fully expects the U.S. to play a greater role in assisting the Iraqi military's effort to retake Mosul.
Chairman of the Joint Chiefs Gen. Joseph Dunford also said that U.S. forces would "do more in Mosul" to help the Iraqi Army reclaim the city -- the country's second-largest -- from ISIS.
Carter also discussed the U.S. cyber campaign against ISIS, using another terms for the organization, saying it was intended to "disrupt ISIL's command and control" and "overload their network," thereby impacting ISIS' ability to control its forces, the local population and the economy.