US President Donald Trump on Sunday called for nations including South Korea, France, China and Britain to help ensure safe passage in the Strait of Hormuz, which Iran has declared closed to US and US-allied traffic.

He later upped the pressure on NATO allies, telling the Financial Times newspaper that the alliance faced a "very bad" future if its members did not do their bit to reopen the strait.

"NATO is an alliance for the defence of territory" and "the mandate to deploy NATO is lacking" in the current situation, Merz's spokesman Stefan Kornelius told a regular press briefing.

Germany needed to know from Israel and the United States "at what point the military objectives in Iran will have been achieved", a foreign ministry spokesman said at the same briefing.

Merz said Friday that the Middle East war must end "as soon as possible" as the conflict "benefits no-one and harms many economically, including us".

At a separate briefing on Monday, Defence Minister Boris Pistorius said Germany wanted all those involved to prevent "further military escalation".

"There will be no military participation" from Germany but Berlin is prepared to support diplomatic efforts to "to ensure safe passage through the Strait of Hormuz", he said.

"We have a situation which we did not provoke... This war started without any consultations," Pistorius added.

Germany's main responsibility is "for the eastern flank and the high north", he said, and "we stay committed to that but we can't be anywhere in the world".

"What does Donald Trump expect from a handful of European frigates in the Strait of Hormuz that the mighty US navy cannot manage alone? This is the question I find myself asking," Pistorius said.