"This is not just an issue of American domestic politics, but it affects the negotiations we are holding in Geneva," Steinmeier told journalists in Washington before meeting with lawmakers on Capitol Hill.
"Obviously, mistrust is growing on (...) the Iranian side if we are really serious with the negotiations."
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Steinmeier added it would be good "if the letter of the 47 senators no longer causes any disturbance in the negotiations."
In the letter to Iran's leaders, the Republicans warned that any deal agreed before Obama leaves office in 2017 is "nothing more than an executive agreement" that could be struck down by Congress later.
The German foreign minister was due to meet the Republican chairman of the Senate Foreign Relations Committee, Bob Corker, and the panel's top Democrat, Senator Robert Menendez.
"Iran's path to the nuclear bomb must be blocked in an unambiguous, verifiable and durable way," Steinmeier said on Wednesday night before meeting with his US counterpart John Kerry for a working dinner.