German Chancellor Angela Merkel tried on Saturday to placate the increasingly vocal critics of her open-door policy for refugees by insisting that most refugees from Syria and Iraq would go home once the conflicts there had ended.

Despite appearing increasingly isolated, Merkel has resisted pressure from some conservatives to cap the influx of refugees, or to close Germany's borders.

Support for her conservative bloc has slipped as concerns mount about how Germany will integrate the 1.1 million migrants who arrived last year, while crime and security are also in the spotlight after a wave of assaults on women in Cologne at New Year allegedly by men of north African and Arab appearance.

The influx has played into the hands of the right-wing Alternative for Germany (AfD), whose support is now in the double digits, and whose leader was quoted on Saturday as saying migrants entering illegally should, if necessary, be shot.

Merkel said it was important to stress that most refugees had only been allowed to stay for a limited period.

"We need to say to people this is a temporary residential status and we expect that, once there is peace in Syria again, once IS has been defeated in Iraq, that you go back to your home country with the knowledge that you have gained," she told a regional meeting of her Christian Democratic Union (CDU) in the state of Mecklenburg-Western Pomerania.

Merkel said 70% of the refugees who fled to Germany from former Yugoslavia in the 1990s had returned.

Horst Seehofer, leader of the Christian Social Union (CSU), the CDU's Bavarian sister party, has threatened to take the government to court if the flow of asylum seekers is not cut.

Merkel urged other European countries to offer more help "because the numbers need to be reduced even further and must not start to rise again, especially in spring".

Fabrice Leggeri, the head of the European Union's border agency Frontex, said a UN estimate, that up to a million migrants could try to come to Europe via the eastern Mediterranean and Western Balkans next year, was realistic.

"It would be a big achievement if we could keep the number stable," he told the magazine Der Spiegel.

Merkel said all EU states should have an interest in protecting the bloc's external borders, and all would suffer if the internal passport-free Schengen zone collapsed and national borders were closed.

AfD leader Frauke Petry told the Mannheimer Morgen newspaper Germany needed to reduce the influx through agreements with neighbouring Austria and a reinforcement of the EU's external borders.

But she also said it should not be shy about turning people back and creating "border protection installations" — and that border guards should, if necessary, shoot at migrants trying to enter illegally.

No police officer wanted to shoot at a migrant, Petry said, adding: "I don't want that either but, ultimately, deterrence includes the use of armed force".

Such comments evoke memories of Germany's Cold War division, when guards in the communist East, led by Erich Honecker, were under orders to shoot people attempting to cross the heavily fortified border into the West.

"The last German politician who let refugees be shot at was Erich Honecker," said Thomas Oppermann, a senior member of the Social Democrats.