German Chancellor Angela Merkel's Christian Democrats (CDU) are set to hold "open-ended" talks with the Social Democrats (SPD) next week, sources told dpa late Thursday, after the center-left party agreed to consider entering into a fresh coalition agreement.

Merkel is expected to lead talks Wednesday evening with SPD boss Martin Schulz and Horst Seehofer, the head of the CDU's Bavarian-based sister party, the Christian Social Union (CSU).

Andrea Nahles, the parliamentary leader of the SPD, said the result of the talks was still open. "It's no more and no less than having talks at the moment," she said, according to Reuters.

The SPD voted Thursday to hold talks with the conservatives after SPD leader Martin Schulz made an impassioned plea for a free hand to work for a social "United States of Europe."

The exploratory talks will be held to determine whether they can foresee working together, before deciding whether to proceed to full-blown coalition talks. SPD leaders will then discuss on Dec. 15 whether formal exploratory talks should begin in January.

Earlier Thursday, after an emotional appeal by Schulz to uphold the national interest and strengthen Europe by backing the talks, a restive party congress in Berlin voted not to rule out hooking up with the chancellor as her junior coalition partner for a third four-year term within 12 years.

"A strong SPD is necessary to make Germany strong, and a strong Germany is necessary to make Europe strong," Schulz said.

SPD officials however said they did not expect to form any new coalition with Merkel until March - six months after the nation went to the polls. Many SPD delegates also criticized Schulz's leadership and spoke out strongly against teaming up with Merkel again. While Schulz was re-elected at the congress as party leader with 81.9 percent of the vote it was sharply down from the 100 percent he received when he was first elected party leader in March.

Merkel is hoping the SPD will agree to a re-run of the "grand coalition" that has governed Germany for the last four years. The collapse of talks between Merkel's conservative bloc, the pro-business Free Democratic Party (FDP) and environmentalist Greens has thrown Germany into political uncertainty and raised the prospect of new elections.

The FDP's shock decision to withdraw from a potential three-way alliance has thrown the government of caretaker chancellor Angela Merkel into chaos, leaving her with only bad choices: Wooing reluctant coalition partners back to the table, running a minority government, or facing new elections. Until a government is agreed, Merkel continues as acting chancellor and previous ministers remain in post.

It has also cast some doubt over whether Merkel, Europe's most powerful leader after 12 years in office, will serve a fourth term after her conservatives bled support to the far-right in a Sept. 24 election, though still won the most seats.

Sitting in the glass-domed lower house of the German Bundestag for the first time were lawmakers of the far-right Alternative for Germany (AfD), the anti-immigration, anti-Islam and anti-Merkel protest party that is at the heart of the crisis.Its entry into parliament in the Sept. 24 elections with almost 13 percent of the vote cost Merkel's conservatives and other mainstream parties dearly, further fragmented the party political landscape and made it harder to gain a parliamentary majority.