The United States stepped up diplomatic pressure on Germany Tuesday to join efforts to secure the strategic Strait of Hormuz as tensions mount between Washington and Iran.
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.
"We've formally asked Germany to join France and the UK to help secure the Strait of Hormuz and combat Iranian aggression," said US 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 US 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 US President Donald Trump regularly criticises Germany for what he considers its insufficient contribution to joint NATO defence.
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.
While other parties have since tried to salvage the accord, the US and Saudi Arabia have accused Iran of being behind multiple mysterious attacks on tankers in the Gulf in June, which Tehran denies.
Iran also shot down an unmanned US aircraft in June, after which Trump announced that he had called off retaliatory air strikes at the last minute because the resulting death toll would have been too high.
- 'Maximum pressure' -
Since then a series of incidents involving oil tankers have heightened tensions.
Britain detained an Iranian tanker off its overseas territory of Gibraltar in early July for allegedly breaching EU sanctions on Syria.
In what many read as a tit-for-tat move, Iran's Revolutionary Guards two weeks later impounded a British-flagged tanker in the Strait of Hormuz.
Britain last week proposed a European-led protection force there, but has since suggested such a mission should involve the United States.
France said last week it was not willing to send extra military assets to the Gulf, but would share information and coordinate its currently deployed assets.
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."