German Chancellor Angela Merkel (L) talks during ARD summer-interview with journalists Tina Hassel and Thomas Baumann in Berlin, Germany, August 28, 2016. REUTERS/Stefanie Loos

BERLIN (Reuters) - German Chancellor Angela Merkel's domestic popularity has declined, a poll showed on Sunday, with 50 percent of Germans against her serving a fourth term in office after a federal election next year.

Support for Merkel has weakened after a string of violent attacks on civilians in July, three of which were carried out by asylum seekers. Of those, two were claimed by Islamic State.

This has raised opposition to Merkel's open-door migrant policy, which allowed hundreds of thousands from the Middle East, Africa and elsewhere into Germany last year.

Half of the 501 people questioned in the Emnid poll for the Bild am Sonntag newspaper were against Merkel staying in office beyond the 2017 election, with 42 percent wanting her to remain.

In November, the last time Bild am Sonntag commissioned a survey on the issue, 45 percent had been in favor of Merkel serving a fourth term, with 48 percent against.

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

When asked about her plans for the 2017 election in an interview with public broadcaster ARD on Sunday, Merkel said she would comment on this "in due course", but did not elaborate.

Germany's political parties are gearing up for next year's election. Asked in the ARD interview when Germans would get tax relief given that Germany has a budget surplus, Merkel said that would come "in the next legislative period".

Finance Minister Wolfgang Schaeuble has previously said that people should not expect much tax relief.

Merkel's Social Democrat coalition partners have promised voters increased spending on infrastructure, education and research.

(Reporting by Michelle Martin and Joseph Nasr; Editing by Raissa Kasolowsky)