The German chancellor said the global fight against terrorism was not an excuse for banning Muslims from the US.
Mr Trump has said that the ban was necessary in preventing “radical Islamic terrorists” from entering the US.
Her spokesman Steffen Seibert said: “She is convinced that even the necessary, decisive battle against terrorism does not justify putting people of a specific background or faith under general suspicion.”
Merkel has also slammed Trump's immigration ban
May meets Trump: Historic first meeting in pictures
Fri, January 27, 2017
The two leaders will spend about an hour in face-to-face talks in the Oval Office, where President Trump has restored a bust of Winston Churchill removed by predecessor Barack Obama.
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President Trump Meets With British PM Theresa May At The White House
Thousands of people have protested against the ban
The 45th president of the US ordered a four-month ban on allowing migrants into the US
Mr Seibert said Mrs Merkel had told Mr Trump during a telephone call on Saturday and reminded him that the Geneva Conventions require the international community to take in war refugees on humanitarian grounds.
He said the German government would now review the consequences of the ban for German citizens with dual nationalities and would "represent their interests, if needed, vis-a-vis our U.S. partner”.
The 45th president of the US ordered a four-month ban on allowing migrants into the US as well as banning travellers from Syria, Iraq, Libya, Iran, Somalia, Sudan and Yemen.
Hollande has also blasted Trump over immigration ban
A spokesman for the Green Party in Germany, Dieter Janecek, said: “Donald Trump is no longer a reliable partner.
“If this insanity is not rescinded, that one should look into whether it is possible to impose a travel ban on him and Stephen Bannon.”
Mr Trump has invited Merkel to Washington soon and Trump accepted Merkel’s invitation to attend the G-20 summit in Hamburg in July .
The US president has said that Mrs Merkel’s lenient refugee policy is a “catastrophic mistake” and has given space for parties such as Alternative for Germany (AfD) to gain ground.
The French President Francois Hollande has urged Europe to form a united front and provide a “firm” response to the US President.