The 85-year-old was admitted to the surgical intensive care ward at the University Clinic of Heidelberg after an operation on his intestines, Spiegel reported.
In the late afternoon, Kohl's office released a statement to Spiegel, saying that his condition was "good considering the circumstances".
But the text neither confirmed nor denied his presence on the intensive care ward.
Bunte magazine, which broke the news of Kohl's admission, reported that he was unconscious for a long time following the operation.
He may be unable to leave the hospital for his planned post-surgery rehab in a Bavarian clinic on the weekend.
A hospital spokesman told Spiegel that a statement might be given later following consultation with Kohl's family.
On May 4th, the aging former statesman's hip was also replaced in an operation at the same clinic.
"I just want to get home as soon as possible," he told Bild at the time.