Pictures from the scene in the Puglia region showed both trains were on the same track about nine miles (15 kilometers) south of the Adriatic Sea, apparently headed toward each other.
Both trains were traveling between 62 to 68 miles per hour (100 to 110 kilometers per hour), ANSA reported.
Images distributed by firefighters there showed the smashed remains of train cars and a large area of scattered, twisted metal at the impact site about 4.3 miles (seven kilometers) east of the city of Andria. A hospital in Andria appealed to the public for blood donations for the injured.
One train was headed from Andria to Corato, while the other was coming in the opposite direction, ANSA reported.
Earlier, ANSA reported a death toll of 27, but the agency changed its count to 23 on Wednesday, citing Puglia regional Gov. Michele Emiliano.
Prime Minister Matteo Renzi promised a full inquiry to find out who was responsible. He met with officials in the region Tuesday afternoon and pledged support, he told reporters in Bari on Tuesday evening.
"My thoughts go to the families of the victims," he said. "We are together in this pain and despair."
The state-run rail company FS Group offered condolences and said both trains were operated by the private company Ferrotramviaria SpA, which runs commuter rail services in the area.
"The employees of the Italian FS Group are close to the victims' families, the wounded and colleagues at Ferrotramviaria," the FS Group said.
Details on what led to the crash weren't immediately available.
But Francesco Giannella, the local public prosecutor, said human error was likely the reason for the collision. At least one person is being investigated in relation to the incident, he said.
Prosecutors have opened an investigation, though no one has been charged with a crime yet.
The wreck happened in a relatively remote area. Local authorities took to social media to ask for any doctors and medics in the area to go to the crash site to help the injured.
"It's not an area that's easily accessible. The closest highway is miles away," CNN contributor Barbie Nadeau said.