BERLIN — The changes ushered in by President Trump mean that Germany, and Europe, must define and defend their own interests and embark on a more independent and assertive foreign policy, Germany’s acting foreign minister said on Tuesday.

The official, Sigmar Gabriel, called for a new willingness to clash with Washington on any range of issues, including trade and Iran.

Mr. Gabriel made his remarks in a speech to a foreign policy conference hosted by the Körber Foundation in Berlin, before he traveled to Brussels, where Secretary of State Rex Tillerson arrived to a cool reception even as he tried to reassure his European partners that Washington remained committed to the trans-Atlantic partnership.

Mr. Tillerson is reported to be on the verge of being fired by Mr. Trump, which the president has denied. Mr. Gabriel’s speech made clear that important allies of the United States remain not only skeptical of the Trump administration but are preparing to part ways on significant issues.