Former German Chancellor Helmut Kohl has been in hospital since early May recovering from hip replacement surgery and a further operation but is doing well and hopes to begin rehabilitation soon, his office said.

Kohl's office issued a statement on his health following reports that the 85-year-old father of German reunification was in intensive care at a clinic in Heidelberg.

Bunte magazine, citing sources close to Kohl, said his plans to leave the hospital in early June were in doubt after he was forced to undergo additional surgery on his intestine.

"Under the circumstances, Dr Helmut Kohl is doing well," his office said, confirming that another operation had been necessary after the hip replacement. "In a next step he plans to go to a rehabilitation centre for his new hip, which he will combine with a vacation."

Germany's longest serving post-war chancellor from 1982 to 1998, Kohl was one of Europe's towering leaders in the second half of the 20th century but he has been physically frail for years and is usually seen in a wheelchair.

At home he is celebrated as the father of German reunification. He also pushed through the introduction of the euro currency together with his close ally French President Francois Mitterand.

In recent years, Kohl has espoused the benefits of closer European integration and voiced doubts about the way his former protege, Angela Merkel, has dealt with the euro zone debt crisis.

His reputation was marred by a financing scandal in his party, the Christian Democratic Union (CDU), that came to light after he left office.