Senior officials from the permanent five members of the UN Security Council and Germany are meeting in London to discuss the next steps over Iran's nuclear programme.
The gathering comes at another crucial juncture in the long-running stand-off, in which Western countries say they suspect Iran of seeking to develop nuclear weapons, and the Iranians repeatedly deny the charge.
Iran's Gulf neighbours are worried about its nuclear programme
As a backdrop, Saudi Arabia has said it and its Gulf allies have proposed as a compromise setting up a consortium with Iran to enrich uranium in a neutral country.
The Americans are being represented at the meeting by the Under-secretary of State for Political Affairs, Nicholas Burns.
He will be pressing the others to move forward on a third UN Security Council resolution on tougher sanctions against Iran, for not suspending its uranium enrichment activities.
New diplomatic phase
This is the first gathering of this key group on Iran since a number of important developments have taken place.
There have been the upheavals in Iran's negotiating team, new US sanctions on Tehran, and top-level visits by Russia's president and foreign minister to the country.
We are also moving into another important phase in the diplomacy.
Iran denies its nuclear work is intended to develop weapons
The International Atomic Energy Agency will make a new report in a couple of weeks on whether Iran has answered all the questions about its past nuclear activities.
The European Union's envoy, Javier Solana, is also expected to give his assessment of the state of play of diplomacy.
And there is the issue of pressing ahead with the new UN Security Council resolution.
The British are behind the Americans.
So, too, are the French, who have moved closer to the United States on sanctions under President Nicolas Sarkozy - perhaps calculating that that is the best way to avoid further escalation.
The Germans, however, remain somewhat more cautious.
The question for this gathering in London is where all this leaves the diplomatic fault lines.
The Americans continue to complain that Russia and China are in effect blocking tougher sanctions.
The US Secretary of State, Condoleezza Rice, has played this down as simply differences over "tactics".
But Moscow and Beijing are clearly uneasy about where this will lead - and continue to advocate more dialogue.
Iranian and UN officials began new talks in Tehran on Monday
However, the level of rhetoric between Washington and Tehran seems as tough as ever.
Against this background, Saudi Arabia says it and its Gulf Arab allies in the Gulf Co-Operation Council are proposing a compromise idea: a consortium that would carry out uranium enrichment for all Middle East countries with nuclear programmes in a neutral location - perhaps Switzerland.
There would be safeguards to avoid the enriched uranium being used for weapons.
The proposal was discussed by the Saudis and British during this week's state visit to Britain by King Abdullah of Saudi Arabia.
Whether this is viable, or even attractive to Tehran, must be open to doubt. A previous offer by Russia for it to enrich Iranian uranium got nowhere.
But it is perhaps a measure of the level of concern now about this stand-off among the Gulf Arab states.
So too may be new remarks by the crown prince of Bahrain, Sheikh Salman Bin Hamid al-Khalifa, to two British newspapers.
He openly accused Iran of developing a nuclear weapons capability, and warning of the risks of a new Gulf conflict.