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.
Mr Schulz told reporters yesterday that he would discuss all options, including selling off his party's very recently acquired principles on a coalition, with SDP leaders on Monday.
In a Facebook Live appearance, Schulz mentioned other possible options for the next government, including a minority administration and a coalition that would include the conservatives, the SPD and the Greens.
German daily Bild reported earlier on Friday that SPD and the Union, comprised of Merkel's Christian Democratic Union (CDU) and its Bavarian sister party Christian Social Union (CSU), has agreed to enter exploratory talks for a renewed coalition talks after the one since the 2013 federal elections. "As before, I remain sceptical that you can do that by carrying on "business as usual" in a grand coalition".
Merkel's party stressed however that was it was seeking is a renewal of its alliance with the SPD, to form a "stable government" for Europe's biggest economy.
"We have many options for forming a government". Schulz fronts a party which is split evenly down the middle on what he should do next. A resolution should then be formed at the party congress in the coming week.
Merkel's camp said the ball was in the SPD's court. By contrast, Steinmeier - himself a former SPD leader - is known to favor the grand coalition option.
"It's now up to the SPD to provide clarity", said CDU manager Klaus Schueler.
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.
"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.