Sources in the SPD said all options would be discussed - ranging from a re-run of the grand coalition with the conservatives to new elections.in Europe's largest economy.
Firmly denying that an agreement had been reached on opening negotiations on a grand right-left coalition, Schulz said that all options remained on the table.
However the SPD is wary of continuing as a junior partner to Merkel since it suffered its worst result ever in September's election. Her attempts to form a three-way tie-up with the pro-business Free Democratic party (FDP) and the Greens failed.
In a sign of the ongoing friction between Schulz and Merkel's bloc that may make negotiations hard, he blamed the Bild report on a leak from her side and said he had telephoned her Friday to say it was "unacceptable".
The SPD will hold a party congress in Berlin on December 7-9, where it is expected to debate its options.
On Friday, her party reiterated that stance, with the CDU's Klaus Schueler saying after a telephone conference of the party leadership: "The CDU is ready to hold serious talks with the SPD to build a stable government".More news: Historic 2017 Hurricane Season ends
Bild daily reported earlier on Friday that the SPD had greenlighted coalition talks after discussions among Schulz, Christian Democratic Union's (CDU) leader Angela Merkel and Christian Social Union (CSU) leader Horst Seehofer on Thursday evening.
However, several leading Social Democrat politicians have supported a coalition or are backing a minority CDU/CSU government.
"The fact that we underlined today that we are prepared to enter such talks with the SPD shows that we're aiming to bring these talks to a successful conclusion", he added.
"Berlin can not perpetually answer suggestions from the French president with a no", he said.
"It's not automatic that there will be a new grand coalition", Schulz told reporters in Berlin.
At the same time, the SPD chief began naming his conditions, stressing in particular that he wants Germany to back French President Emmanuel Macron's initiative to reform the European Union.