Britain, France, Germany, Italy, Japan and the Netherlands said Thursday they were ready "to contribute to appropriate efforts to ensure safe passage through the Strait of Hormuz".

The grouping said they "welcome the commitment of nations who are engaging in preparatory planning", as they condemned "in the strongest terms recent attacks by Iran on unarmed commercial vessels in the Gulf".

But Italy and Germany both made clear later Thursday that they were not talking about any immediate military help, but rather a potential multilateral initiative after a ceasefire.

The declaration came as an effective Iranian blockade of the strait has paralysed commercial shipping through the crucial maritime chokepoint, which in peacetime sees a fifth of global crude oil and liquefied natural gas pass through it.

The war, which erupted on February 28 when the United States and Israel began bombing Iran, has led Tehran to retaliate with strikes across the Gulf region.

Twenty-three commercial vessels, including 10 tankers, have reported incidents or having been attacked.

The situation has left around 20,000 seafarers stranded on approximately 3,200 vessels west of the strait, according to the International Maritime Organization.

"We express our deep concern about the escalating conflict," the allies' joint statement said.

"We call on Iran to cease immediately its threats, laying of mines, drone and missile attacks and other attempts to block the Strait to commercial shipping," it added.

"Freedom of navigation is a fundamental principle of international law, including under the United Nations Convention on the Law of the Sea.

"The effects of Iran's actions will be felt by people in all parts of the world, especially the most vulnerable."

- Not a 'war mission' -

US President Donald Trump has urged other world powers, and NATO, to help reopen the Hormuz Strait to commercial shipping.

But they have rebuffed his call in the short term while insisting they were open to discussions and planning.

Italy's Defence Minister Guido Crosetto said the statement by the six countries should not be seen as a "war mission".

"No entry into Hormuz without a truce and a comprehensive multilateral initiative", for which "it is right and appropriate for the United Nations to provide the legal framework", he said in a statement.

And in Berlin, Defence Minister Boris Pistorius said that any German military involvement "would depend on the situation after a ceasefire... and whether we could participate within the framework of an international mandate".

Military involvement would also require approval by the German parliament, he added.

A UK defence official told reporters at a briefing Wednesday that "the level of threat is such that I don't see many nations being willing to put warships into the middle of that threat right now".

The defence official noted London has sent a "small number" of additional military "planners" to US Central Command to "help with the planning and option development for... whatever comes next in the Strait of Hormuz might look like".