The talks in Sofia from Thursday come as Iran stands firm ahead of a Friday deadline set by the UN Security Council for Tehran to freeze uranium enrichment work as a "confidence-building measure."

NATO is not directly involved in the Iran crisis, but the talks -- joined by European Union colleagues Thursday and Russian counterpart Sergei Lavrov Friday -- will provide an obvious opportunity to take stock.

"I would guess a lot of them are thinking about what happens if this deadline passes without a change of heart from the Iranians," said a senior NATO diplomat.

"NATO is concerned by the issue, but it is not playing a leading role," added NATO chief Jaap de Hoop Scheffer, who will chair the talks in the Bulgarian capital.

The stakes were raised further this week after Iran's hardline President Mahmoud Ahmadinejad warned that the Islamic republic could quit the Non-Proliferation Treaty if UN sanctions are imposed on it.

The official agenda of the informal Sofia talks notably includes discussion of Darfur, as well as preparations for a NATO summit in the Latvian capital Riga in November, officials say.

Pressed by Washington, NATO has said it is ready to increase its support for an African Union-led peacekeeping force in the violence-scarred Sudanese region, as well as for a UN force expected to replace it later in the year.

Intense discussion are underway between NATO's 26 member states' governments over how exactly to do that, without actually sending combat troops into the region.

The Bulgarian talks will also provide a chance to take stock of NATO's expansion in Afghanistan, where it has led the International Security Assistance Force (ISAF) since 2003.

But the planned ISAF advance into the volatile south of the country -- where it will take over from a US-led operation focused on fighting insurgents -- is being clouded by a surge in violence, including attacks on NATO troops.

The Sofia meeting will be the last such talks ahead of a November 28-29 summit in Latvia, where NATO leaders will notably discuss expansion of the alliance further into ex-communist lands.

The United States and Britain are also keen to bolster ties between the West's former Cold War military bloc and other countries which share its values, such as Australia, New Zealand and Japan.

"This is not about making NATO into a global alliance, it is about making NATO into a global partnership," said de Hoop Scheffer, adding that Sofia "will be an important building bloc for the Riga summit."

But almost inevitably the Iran crisis will overshadow the talks, which will come a day after last-minute talks Wednesday between the head of Iran's nuclear agency and the United Nations nuclear watchdog, two days ahead of the deadline.

US Secretary of State Rice, travelling in Europe ahead of the Sofia talks, on Tuesday urged the Islamic Republic to abide by international demands for a diplomatic resolution of the conflict.

"We ... have to be concerned when there are statements from Iran, as there were apparently today, that Iran would not only have this technology but also would share its technology and expertise," she said in Ankara.

"This is about not allowing Iran to get the expertise and the technology to build a nuclear weapon, which Iranian leaders from time to time say they would gladly transfer to others," Rice said.

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