News outlet Politico, which did not name its sources, said the meeting was coordinated with Kyiv and that Schroeder had travelled to Russia via Turkey.
The German government led by Chancellor Olaf Scholz, who like Schroeder is a Social Democrat (SPD), was not informed about the trip, government sources told AFP.
Bild daily, also citing unnamed sources, said that Schröder had told only his closest confidants about the meeting.
SPD co-leader Lars Klinbeil said on broadcaster ZDF that his party did not know anything about the trip, but added that “anything that helps to end this terrible war is welcome.”
Schröder’s wife Soyeon Schröder-Kim posted late Thursday on Instagram a photograph of herself with eyes closed and hands clasped like in a prayer, with the Kremlin in the background.
Schröder, who was Germany’s chancellor from 1998 to 2005, has come under fire at home and abroad over his refusal to resign from his job as chairman of the board of directors at Russian oil giant Rosneft despite Moscow’s assault of Ukraine.
The 77-year-old is also chairman of the shareholders’ committee at Gazprom’s subsidiary Nord Stream, and is due to join the supervisory board of Gazprom itself in June.
Schröder has issued a statement condemning the invasion as unjustified but saying that dialogue must continue with Moscow.
Furious over his refusal to cut his Russian ties loose, his aides have walked out on him.
Scholz has also come out publicly to urge Schröder to leave his Russian jobs, saying it is “not correct” for him to hold those offices.