Under the plan, which has only been finalised in recent days, the United States would provide command ships and lead surveillance efforts for the military coalition.

Allies would patrol waters near those US command ships and escort commercial vessels with their nation's flags.

Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, articulated those details to reporters following meetings on Tuesday with acting US Defence Secretary Mark Esper and Secretary of State Mike Pompeo.

"We're engaging now with a number of countries to see if we can put together a coalition that would ensure freedom of navigation both in the Straits of Hormuz and the Bab al-Mandab," Dunford said.

"And so I think probably over the next couple of weeks we'll identify which nations have the political will to support that initiative and then we'll work directly with the militaries to identify the specific capabilities that'll support that."

Iran has long threatened to close the Strait of Hormuz, through which almost a fifth of the world's oil passes, if it was unable to export its oil, something US President Donald Trump's administration has sought as a way to pressure Tehran to renegotiate a deal on its nuclear program.

But the US proposal for an international coalition to safeguard shipping in the Strait, at the mouth of the Gulf, has been gaining momentum since attacks in May and June against oil tankers in Gulf waters.

A spokeswoman for Australia's Defence Department said on Wednesday the government was "deeply concerned by heightened tensions in the Gulf".

"We strongly condemn the recent attacks on shipping in the Gulf of Oman. Australia relies on freedom of navigation and the uninterrupted passage of maritime trade," the spokeswoman said.

"As a matter of course, Australia consults closely with our allies and partners on the situation in the Middle East. We are following the situation closely, including through our embassies in the region."

The deputy chief cabinet secretary of Japan, one of Washington's key global allies, declined to comment directly when asked about Dunford's comments.

"We are quite concerned about mounting tensions in the Mideast, and guaranteeing safe passage in the Hormuz Strait is vital to our nation's energy security, as well as to the peace and prosperity of international society," Kotaro Nogami told a regular news conference in Tokyo on Wednesday.

"Japan will stay in close contact with the United States and other related nations and continue to make efforts for stability and the reduction of tension in the Mideast."

Although US officials had publicly discussed plans to safeguard the Strait, Dunford's disclosure that the coalition would also seek to bolster security in the Bab al-Mandab off Yemen appeared to be a new element.

The US, as well as Saudi Arabia and the United Arab Emirates, have long fretted over attacks by Iran-aligned Houthi fighters in the narrow Bab al-Mandab waterway, which connects the Red Sea with the Gulf of Aden and the Arabian Sea.

Australian Associated Press