Frankfurt am Main (AFP) - Angela Merkel's junior coalition partners threatened Sunday to quit her governing coalition, blaming bitter infighting in Berlin for massive losses to both the chancellor's conservative party and the centre-left in a regional vote.

"The state of the government is unacceptable," Social Democratic Party (SPD) general secretary Andrea Nahles said at a Berlin news conference.

After leaders reluctantly agreed to a third right-left coalition earlier this year, the fourth Merkel government has staggered from one crisis to another, almost collapsing twice over the summer in battles over relatively minor points.

On Sunday, Merkel's conservative Christian Democratic Union (CDU) suffered a bruising 10-point blow from voters in Hesse state, garnering 28 percent of the vote according to an exit poll from broadcaster ARD.

But the SPD, her reluctant partners in Berlin, came off even worse, shedding almost 11 points to a historical low of 20 percent.

It was the latest in a string of disappointments for the SPD, Germany's oldest political party, with many members blaming repeated alliances with Merkel for its decline.

Berlin must find a "reasonable way of working", Nahles said, arguing that the dismal picture the two parties were painting in the capital had "significantly" contributed to the regional election blow.

The CDU must now agree to a "clear, binding roadmap for politics in the interest of the citizens," she demanded, ahead of a "half-time review" next September.

"Then we'll be able to check whether this government is still the right place for us," Nahles said -- implying the SPD may quit if Merkel does not offer new concessions.

Nahles' room for manoeuvre is nevertheless limited, as quitting the government and triggering new elections would likely prove even more painful for the SPD than for the CDU according to current polls.