Half of all Germans believe Angela Merkel should not serve a fourth term as chancellor, as a wave of criticism hits her open-door migration policy.

A series of attacks in Germany on civilians last month, including two claimed by terror group the Islamic State (Isis), have led to calls from opposition politicians and some citizens for Merkel to halt her government's policy that has seen hundreds of thousands of refugees enter the country.

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An Emnid poll of 501 people for Bild am Sonntag newspaper found that 50% were against Merkel staying in office beyond the 2017 election, with 42% wanting her to remain.

Another poll on the same question in November saw 45% in favour of Merkel remaining in power, with 48% against.

It comes as the head of Germany's Federal Office for Migration and Refugees (BAMF), Frank-Juergen Weise, said he expects a maximum of 300,000 refugees to arrive in Germany this year.

He claimed original estimates that 1.1m migrants and refugees had entered the country last year were incorrect, saying it was fewer than one million.

Sigmar Gabriel, Germany's vice chancellor, said the influx was still a record number and that Merkel had "underestimated" the challenge of integrating new arrivals and finding them jobs.

He also criticised Merkel's "Wir schaffen das" ("We can do this") slogan, which she repeatedly used in reference to managing the migrant crisis.

Merkel, 62, has yet to reveal whether she will seek re-election next year, saying: "I will comment on that at the appropriate time. I'm sticking to that."

Support within Merkel's center-right Christian Democratic Union (CDU) party remains strong, with 70% in favour of her serving another term, and 22% opposed.

She is waiting to see if she has the backing of the CDU's Bavarian sister party, the Christian Social Union (CSU), before announcing her desire to run again, according to German magazine Der Spiegel.

Should Merkel be re-elected, she could become the country's longest-serving chancellor, with the current record of 16 years held by Helmut Kohl, who served from 1982-1998.

The next German federal elections are expected to be held between August and October next year.