European Commission chief Jean-Claude Juncker on Monday congratulated Angela Merkel on a "historic" German election victory but called for the formation of a strong coalition government as soon as possible.

Merkel met with her party Monday after winning a fourth term with a far weaker mandate, facing the double headache of an emboldened nationalist opposition party and thorny coalition talks.

"In the light of the important global challenges, Europe needs a strong German government now more than ever, one able to actively shape the future of our continent," his spokesman Margaritis Schinas quoted Juncker as telling Merkel in a letter.

"And in his letter president Juncker expresses his belief that the negotiations on the coalition government will contribute to that effect."

Juncker also spoke by telephone to Merkel to "congratulate her warmly for her historic victory for the fourth time in a row," Schinas added.

Germany is the EU's most populous country and biggest economy, and Merkel has taken a leading role dealing with a series of crises from eurozone debt to Brexit. (AFP)