German Chancellor Angela Merkel will meet leaders of the Social Democrats next Wednesday after the labour party gave the green light to exploratory talks on ending the country's political stand-off.

SPD parliamentary group leader Andrea Nahles told German radio on Friday that she would join the meeting along with SPD leader Martin Schulz, who has called for a United States of Europe "by 2025".

The head of Merkel's Bavarian sister party, Horst Seehofer, will also attend the talks along with Nahles' conservative counterparts in parliament.

At a fractious party congress on Thursday, Schulz won members' approval for "open-ended" talks that could lead to either a new right-left "grand coalition", a Merkel minority government or, if negotiations fail, new elections.

Ten weeks after an inconclusive general election left German politics deadlocked, Schulz had pleaded with his party to give its blessing for such talks, promising to push key demands including improving social welfare and strengthening the European Union.

Schulz, the former president of the European Parliament, vowed to extract a high price if the SPD supports Merkel's bid to lead Europe's biggest economy for a fourth term from early 2018.

Schulz was later reelected party chief with almost 82 percent of the vote.

The SPD motion said the party was "aware of its responsibility to our country" and "not indifferent" to whether a new government is formed, but pledged that any government that emerges must drive forward "the fundamental renewal of Europe".

Nahles said the SPD would start the negotiations in a constructive spirit.

"You don't go into talks with a giant rucksack full of red lines -- otherwise you can forget about negotiations."

Source: AFP