German Chancellor Angela Merkel's coalition partners on Monday accused her of running out of ideas after she announced she would stand for a fourth term in next year's parliamentary election.

The center-left Social Democrats (SPD), junior partner to Merkel's conservatives in their ruling "grand coalition", accused the chancellor of a "pretty feeble performance" in announcing her candidacy on Sunday evening.

By saying she will seek a fourth term, Merkel has effectively fired the starting gun on campaigning for the election in September next year, though she must govern with the SPD until then.

"I can't see how Angela Merkel wants to improve this country and prepare it for future challenges," SPD Secretary General Katarina Barley told reporters, adding that Merkel had run out of steam after 11 years in office.

"I think Mrs Merkel has her merits, but she doesn't stand for the future anymore," said Manuela Schwesig who sits in Merkel's cabinet as minister of family affairs. The SPD politician was answering a question whether she experienced Merkel as being feeble in the cabinet, too.

Merkel, 62, said on Sunday that next year's election would be more difficult than any since reunification in 1990 due to the threat from the right, social divisions and the possibility that Germany's leftist parties could form a coalition.

An Emnid poll on Sunday put support for Merkel's conservative bloc down 1 point at 33 percent, 9 points ahead of her nearest rivals, the Social Democrats.

Barley reaffirmed the SPD's position that it would announce its nominee for chancellor at the end of January. Candidates under discussion are SPD chairman Sigmar Gabriel and the European Parliament's president, Martin Schulz.

Although her conservatives have lost some support at home, Merkel remains respected abroad for her management of the euro zone crisis and overseeing Europe's absorption last year of the biggest influx of migrants to the continent since World War Two.

U.S. President Barack Obama last week described her as an "outstanding" ally.