Speaking at a news conference after a key party executive meeting in Berlin, Martin Schulz sharply criticized Christian Democrats for leaking information to the media, after a crisis summit hosted by President Frank-Walter Steinmeier on Thursday.
Schulz told reporters Friday that they'd agreed to keep their discussions confidential, but that he was now prepared to discuss with his party's leadership on Monday all options, including re-forming a grand coalition.
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.
"It's now up to the SPD to provide clarity", said CDU manager Klaus Schueler, adding that the conservatives respected the approach of SPD party leader Martin Schulz to first discuss how to proceed at a party congress next week.
Her efforts to create a three-way government with the liberal FDP and Greens failed after weeks of negotiations following the election.
He added that whoever circulated such reports was damaging trust.
Schulz said a grand coalition was "not automatic" and that the SPD party convention would also discuss other alternatives.
"We have many options for forming a government".
"Another senior member of Merkel's Christian Democrat Union (CDU), Mike Mohring, said he was hopeful for an eventual grand coalition and expected a new government to be formed by March".
Schulz previously opposed a "grand coalition" with the CDU/CSU, arguing that the election result meant voters had rejected the option of another grand coalition between the SPD and the CDU/CSU.
"Germany's European politics must change", Mr Schulz, a former president of the European parliament, told the magazine. "And the initiatives the French president has taken must not be answered with a constant "no" or with silence from Berlin", he said.
"Giving Emmanuel Macron a positive answer will be a key element in every negotiation with the SPD", Schulz was quoted as saying in the interview made available on Friday, adding that he also backed a joint European Union tax policy.