The request comes 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 a statement by embassy spokeswoman Tamara Sternberg-Greller.

"Members of the German government have been clear that freedom of navigation should be protected... Our question is, protected by whom?"

Long-simmering tensions have spiked between Tehran and Washington since US President Donald Trump unilaterally withdrew from the Iran nuclear deal last year and reimposed biting sanctions on the Islamic republic.

The US and Gulf powerhouse Saudi Arabia have since accused Iran of being behind multiple mysterious attacks on tankers in the Gulf in June, which Iran 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.

Since then a series of incidents involving oil tankers have heightened tensions.

The US request to NATO ally Germany is highly controversial in the country, where many politicians fear any naval mission, especially one led by the United States, could heighten the risk of conflict and drag European powers into a war.

Berlin has been clear it rejects Trump's strategy of "maximum pressure" on Iran.

Britain detained an Iranian tanker off its overseas territory of Gibraltar in early July on allegations it was 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 said last week it was planning a European-led protection force there, but has since suggested such a mission should involve the United States.