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During a visit to Hannover, the US president said: “She is on the right side of history on this.

"In this globalised world, it is very difficult for us to simply build walls.”

His remarks come amid an increasingly fractious debate over radical Islamism in Germany, sparked by Mrs Merkel's ill-fated open door asylum policy.

Shocking opinion polls delivered a crushing blow to the German Chancellor as it was revealed Mrs Merkel's conservatives lost in two out of three state elections.