The 85-year-old conservative elder statesman, known as the father of German reunification, underwent an intestinal operation at the university clinic of Heidelberg, glossy celebrity magazine Bunte and news outlet Spiegel Online reported.
Kohl has been in the intensive care ward for three weeks and is in a "critical" condition, Spiegel added.
The reports could not be immediately confirmed by AFP.
Kohl, who was chancellor from 1982 until 1998, oversaw his country's peaceful reunification the year after the 1989 fall of the Berlin Wall and the end to the Cold War.
He exited active politics in 2002 and has been recently confined to a wheelchair with his speech severely impaired after his jaw was paralysed in a fall.
