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.
Noting that his party and Merkel's bloc continue to govern as a caretaker government, he said "we have no time pressure" and would "not rule out any options".
"It's not automatic that there will be a new grand coalition", Schulz told reporters in Berlin.
Other options apart from a grand coalition include a minority conservative government - which the SPD could support on a case-by-case basis, or fresh elections.
"Whoever circulates false reports destroys trust", he said.
The chairman said the SPD's executive committee would discuss various options on Monday, and make a proposal which would be discussed at SPD's party conference from December 7 to 9 in Berlin.
Merkel, her own political future on the line after 12 years at the helm, is making overtures to the centre-left SPD - her partner in government over the past four years - after her bid to form a three-way coalition with two smaller parties failed.
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Pakistan has been an observer with the SCO since 2005 and applied for its full membership in 2010. The summit is going to be attended by leaders from Pakistan and China.
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However, Schulz did not slam the door for a renewed SPD-Union partnership, saying the SPD leadership was keeping its options open. A resolution should then be formed at the party congress in the coming week.
Merkel this week said she hoped to talk with the Social Democrats "in a serious, engaged, honest way and obviously with the intention of success".
Martin Schulz, the leader of the rival Social Democrats (SPD), told an interview with Spiegel magazine he would insist on deeper integration as a condition of joining any new government under Mrs Merkel.
"The decisive question is what do we want to implement, what can we implement - for example in the renewal of the European Union", Schulz said.
In particular, he said he would demand German support for Emmanuel Macron's proposals for a Brussels-based finance minister and single budget for the Eurozone.
The unusually testy tone Schulz struck is a reflection of the differences between conservatives and Social Democrats not just over the policies of a possible grand coalition, but whether even to try to form one at all.