Chancellor Angela Merkel's government was responding to a US demand made public a day earlier "to help secure" the world's busiest oil shipping lane and "combat Iranian aggression".

At a time of heightened tensions, Berlin fears that this could potentially draw it and other powers into a military confrontation between the United States and the Islamic Republic.

Government spokeswoman Ulrike Demmer said Germany continued to support diplomatic efforts to ease tensions that have risen sharply since US President Donald Trump last year withdrew from the Iran nuclear deal.

"The USA recently presented their concept for a Persian Gulf maritime surveillance mission to several allies including Germany and asked for contributions," said Demmer.

"The German government is reluctant about the concrete US proposal and has therefore not offered a contribution, as the overall approach of our policy toward Iran differs significantly from the current US approach."

Berlin was seeking an emphasis on "diplomacy and de-escalation" and to save the nuclear deal, Demmer said, stressing that "participation in a US-led mission could complicate this issue, even as of course we share the goal of freedom of navigation".

Britain last week ordered its navy to escort UK-flagged ships in the strait in response to Iranian soldiers seizing a tanker in the flashpoint entrance to the Gulf.

Germany remained "in close coordination with France and Britain" on questions of maritime security, Demmer said, adding that Berlin believed the idea of a European naval mission was "worth considering".