NATO chief Jaap de Hoop Scheffer voiced concern about Tehran's nuclear ambitions even as ministers arrived in the Bulgarian capital Sofia.

"Of course, the NATO allies are as worried as ... I am about nuclear proliferation," citing the threat of chemical and biological weapons. "I think I can safely say Iran will be a subject of conversation at dinner tonight."

Gathered for regular talks, the NATO chiefs were set to focus officially on issues including beefing up support for peacekeepers in Darfur and expanding a NATO-led force into more volatile parts of Afghanistan.

But the discussions risk being overshadowed by a UN deadline Friday for the Islamic republic to freeze uranium enrichment work, with little or no sign of movement from Tehran.

Iran's supreme leader upped the stakes in the crisis Wednesday, warning the United States it would be "harmed" across the globe if it decided to attack the Islamic republic over the disputed nuclear programme.

NATO stresses that it has no formal role in the crisis. But the situation will inevitably be discussed. "Although it is not playing the first violin, what happens there is a very NATO-relevant subject," de Hoop Scheffer said.

The first session of talks started in Sofia shortly after the arrival of the ministers including Rice, who flew in from Iraq where she made a surprise visit on Wednesday.

On Darfur, NATO has said it is ready to extend and 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.

Officials stress that a decision on prolonging training and other help beyond May 31, when the current mandate expires, depends on being asked by the African Union. "We will not impose ourselves," said spokesman James Appathurai.

The ministers were also set to take stock of NATO's planned expansion in coming months into the dangerous south of Afghanistan, where it has led the International Security Assistance Force (ISAF) since 2003. The expansion will see the current 9,000-strong force beefed up to 16,000 troops.

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

Ukraine's NATO hopes will also be on the agenda, although the spokesman played down a press report this week that Kiev could soon be offered a so-called membership action plan, which would be the clearest green light yet.

But inevitably the Iran crisis will overshadow the talks, which come after no progress was reported at a last-minute meeting Wednesday between a top Iranian official and the UN nuclear watchdog, the International Atomic Energy Agency.

On Thursday, Iran's hardline President Mahmoud Ahmadinejad vowed that his country "will not bow to injustice and pressure," the day before the expiry of a UN deadline to suspend sensitive nuclear work.

"Thanks to God, we are a nuclear state. We want peace and security and we are not a threat to anyone. We are ready for dialogue on disarming the big powers in order to reinforce peace and security," he added.

Security is tight for the Sofia talks, with some 3,000 police officers on the streets and around the National Palace of Culture, which will host the meeting.

About 1,000 nationalist protestors rallied in Sofia against the visit by the US secretary of state, who is to sign accords to set up three military bases in Bulgaria, brandishing banners including one reading: "Yankees Go Home".

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.