Chancellor Angela Merkel on Saturday moved to sharpen an ongoing debate in Germany over Islam and migration, by saying that "multiculturalism has absolutely failed" and that all immigrants must learn fluent German, DPA reported.
Merkel's comments came a day after under-fire conservative leader Horst Seehofer proclaimed that "multiculturalism is dead" at a youth rally.
Merkel's conservative Christian Democrats have made integration - foreigners adapting to the German way of life - a key policy plank, in opposition to multiculturalism, which prescribes no dominant culture.
Merkel said that Germany had to not only help immigrants, but also challenge them. This had not happened enough in the past, she said.
"Without being able to speak German, integration in Germany cannot work," Merkel told a conference of young Christian Democrats (CDU) in Potsdam.
Seehofer, leader of the Bavarian sister-party on which Merkel's CDU relies to keep power, has come under fire for saying that Germany should not accept any more immigrants from "alien cultures" such as Turkey and Arab states.
Islam has been singled out by some conservatives as being behind Germany's sometimes problematic relationship with its 4 million Muslims, most of whom are from Turkish backgrounds.
In an interview with the Sueddeutsche Zeitung daily on Saturday, Turkish President Abdullah Gul said that Turks in Germany should learn German "fluently, and without having an accent."