DUBAI  - The United States and its allies have launched a major naval exercise in the Gulf that they say shows a global will to keep oil shipping lanes open as Israel and Iran trade threats of war.

Publicly announced in July, the operation, known as IMCMEX-12, focuses on clearing mines that Tehran, or guerrilla groups, might deploy to disrupt tanker traffic, notably in the Strait of Hormuz, between Iran and the Arabian Peninsula.

The start of the event, with a symposium for officers from more than 30 navies, came as Israeli Prime Minister Benjamin Netanyahu told US television viewers on Sunday that Tehran was close to being able to build a nuclear bomb; his words fuelled talk of an Israeli strike, and of Netanyahu pressuring President Barack Obama to back Israel as Obama battles for re-election.

Military officials, diplomats and analysts - as well as Iran itself - all sought to play down the significance of the timing and to stress the defensive and hypothetical aspects of the exercise, which moves on to the water from Thursday with ships from a much smaller number of nations taking part in manoeuvres. However, it was a clearly deliberate demonstration of the determination on the part of a broad coalition of states to counter any attempt Iran might make to disrupt Gulf shipping in retaliation for an Israeli or US strike on its nuclear facilities - a form of retaliation Iran repeatedly threatened.

“This exercise is about mines and the international effort to clear them,” Vice Admiral John Miller, commander of the US Naval Forces Central Command, told officers assembled for the symposium at his fleet headquarters in Bahrain on Monday. “Represented here are the best of our individual countries’ efforts dedicated to securing the global maritime commons.”

As well as Britain and France, the main European naval powers, a number of Middle Eastern states are taking part, along with countries from as far apart as Estonia and New Zealand.

“The demining efforts are clearly in preparation for a showdown with Iran,” said Hayat Alvi of the US Naval War College, “Presumably in the context of either an Israeli strike targeting Iran’s nuclear facilities, or some provocation that leads to an Iranian response in the Persian Gulf region.”

US forces in the Gulf include two aircraft carriers on permanent station, though these will not take part in the latest exercise - one of dozens held by the fleet every year. For its part, Iran has said it will hold a major air defence exercise next month, showing its ability to protect nuclear sites.

Western powers are also involved in planning a major naval exercise to be held in the eastern Mediterranean next month.

Netanyahu’s on US television stressed the notion that Tehran was on the brink of a major breakthrough which Israel would not accept.

Catherine Ashton, the European Union foreign policy chief who is leading international efforts to persuade Iran to accept checks and limits on its nuclear work, will meet the chief Iranian negotiator Saeed Jalili in Istanbul on Tuesday, officials announced on Monday. The encounter follows a renewed round of discussions begun in May after over year of stalemate.

With strains showing between Israel and Washington, German Chancellor Angela Merkel, a powerful ally to both, said on Monday that she believed a peaceful outcome was still possible, despite scepticism from Netanyahu that diplomacy will work.

“Iran is not just a threat to Israel but for the whole world,” Merkel told a news conference, in remarks that echo an element of Israel’s arguments. “I want a political solution and think we should act together internationally, and I believe that the room for a political solution has not been exhausted.”

But Iranian military officials sounded a relaxed note, reassuring their own public: “This exercise is a defensive exercise and we don’t perceive any threats from it,” Mohammad Ali Jafari, commander of the Islamic Revolutionary Guard Corps, was quoted as saying on Sunday. “We are not conducting exercises in response.”

Anthony Skinner of the Maplecroft consultancy said the exercise would, however, remind Iran of Washington’s ability to blunt its offensive capabilities: “Iran would likely mine the Strait of Hormuz and possibly deploy suicide bombing skiffs in the event of air strikes against its nuclear facilities.

“Washington wants to show that it’s prepared for such an eventuality,” Skinner said. “I see this exercise as part of broader initiative to sustain the pressure on Iran. Giving sanctions the time to work is clearly the preferred option for the Obama administration, at least in the medium term.”