BERLIN (Reuters) - German Vice Chancellor Sigmar Gabriel said in an interview on Saturday that Chancellor Angela Merkel's conservatives had "underestimated" the challenge of integrating a record migrant influx.

Gabriel is also leader of the Social Democrats (SPD) - the junior coalition partner in Merkel's government - and his comments to broadcaster ZDF come as campaigning gets under way for a federal election next year.

More than a million migrants flocked to Germany from the Middle East, Africa and elsewhere last year. Concerns about how to integrate them all into German society and the labor market are now rife and support for the anti-immigrant Alternative for Germany (AfD) has grown.

"I, we always said that it's inconceivable for Germany to take in a million people every year," Gabriel said in extracts of an interview released on Saturday.

The IAB German labor office research institute says around 16,000 are still arriving each month, compared with more than 200,000 in November.

Gabriel also criticized Merkel's catchphrase "Wir schaffen das", meaning "We can do this", which she adopted during the migrant crisis last summer and has repeatedly used since.

Merkel used the phrase at a news conference she held in late July after a spate of attacks on civilians in Germany, including two claimed by Islamic State, that have put her open-door migrant policy in the spotlight. Her popularity has slipped since those attacks.

Gabriel said repeating that phrase was not enough and the conservatives needed to create the conditions for Germany to be able to cope, adding that the conservatives had always blocked opportunities to do that.

(Reporting by Michelle Martin, editing by Larry King)