The United States has stepped up diplomatic pressure on Germany to join efforts to secure the strategic Strait of Hormuz as Iran has detained a British oil tanker and other vessels face a risk.
The request came after Britain last week ordered its navy to escort UK-flagged ships in the world's busiest oil shipping lane in response to Iranian soldiers seizing a tanker in the flashpoint entrance to the Gulf.
Reuters reported on Tuesday that British Petroleum has decided not to use its own tankers for carrying oil through the Strait of Hormuz and is using vessels flagged by other countries.
"We've formally asked Germany to join France and the UK to help secure the Strait of Hormuz and combat Iranian aggression," said U.S. embassy spokeswoman Tamara Sternberg-Greller in a statement.
"Members of the German government have been clear that freedom of navigation should be protected... Our question is, protected by whom?"
The U.S. request is highly controversial in Germany, where many politicians fear any naval mission, especially one led by the US, could heighten the risk of conflict with Iran.
It comes at a time when U.S. President Donald Trump regularly criticizes Germany for what he considers its insufficient contribution to joint NATO defense.
Long-simmering tensions have spiked between Tehran and Washington since Trump unilaterally withdrew from the Iran nuclear deal last year and reimposed biting sanctions on the Islamic republic.
A German foreign ministry source said that the US had "recently presented its concept for a maritime surveillance mission ... to a number of allies, including Germany, and asked for contributions".
"The German government has taken note of this, but not promised to make any contribution."
Foreign Minister Heiko Maas had "repeatedly stressed that, in our view, the priority must be to de-escalate tensions and continue with diplomatic efforts," said the ministry source.
"We are working closely with France and the United Kingdom on this. We can rule out participation in the American strategy of maximum pressure."
Reporting by AFP and Reuters
