German Foreign Minister Sigmar Gabriel on Thursday (AEDT) announced plans to visit Washington and shore up ties with Germany's closest ally outside Europe, days after a key aide to US President Donald Trump launched fresh attacks on Berlin's policies.

Gabriel said he looked forward to a "good, open and friendly" dialogue with Rex Tillerson, confirmed as Trump's secretary of State on Thursday by the US Senate, and said Germany was seeking answers about the new US administration's foreign policies, its relationship to the NATO alliance and other key issues.

"The world will not wait for us. There are urgent issues on the global agenda about which Germany and America, as well as Europe and America, should be closely coordinating," Gabriel said in a statement.

"The friendship between two countries is far more than a beneficial cooperation between governments, but without good and trustworthy relations between those governments, it will not go well," said Gabriel, who also serves as vice-chancellor.

German Chancellor Angela Merkel was the go-to European ally for former US President Barack Obama, who praised her as "an outstanding partner" on a farewell visit to Berlin in November, but the relationship has deteriorated markedly under Trump.

Last month, Trump said Merkel made a "catastrophic mistake" with her open-door refugee policy. This week, his top trade adviser said Germany was using a "grossly undervalued" euro to gain advantage over the United States and its European partners .

Merkel's spokesman Steffen Seibert, asked about attacks from Washington, described the German-American relationship as a deep "historical friendship between two peoples".

Pressed to comment about Trump's attacks on Germany's migrant and trade policies, and the euro, Seibert told a regular government news conference: "We are at the very beginning of the cooperation with a new American government."

Ingo Kramer, president of the Federation of German Employers, told industry executives that Trump's actions and words were unsettling, but he hoped that Washington would not continue with its "retreat from the rest of the world".

Gero Neugebauer, a Berlin-based political expert, said a barrage of criticism from Trump had forced Merkel to abandon her plan to refrain from public remarks about the US President.

"Merkel has no choice but to step into the breach and stand up for German interests regardless of how great her desire for cooperation," he said.

Merkel's government has offered to visit the US in the spring in her capacity as chairman of the G20 group of leading economies, government sources have said.

Trump has accepted an invitation to go to a G20 summit that Merkel will host in Hamburg in early July, and on Saturday said he looked forward to welcoming Merkel to Washington.

The pair issued a positive statement after speaking by telephone on Saturday about NATO, the situation in the Middle East and North Africa, their ties to Russia and the conflict in eastern Ukraine.

But Merkel later sharply criticised Trump's travel ban on refugees and people from Muslim-majority countries.