Six major international powers, including Britain, France, Germany and Japan, said Thursday they were ready "to contribute to appropriate efforts to ensure safe passage through the Strait of Hormuz".

"We welcome the commitment of nations who are engaging in preparatory planning" the grouping -- which also includes Italy and the Netherlands -- said in a joint statement, as they condemned "in the strongest terms recent attacks by Iran on unarmed commercial vessels in the Gulf".

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.