Straw, who was finishing a two-day visit to Iran on Monday, told BBC radio that Iran could not be compared with its neighbour Iraq in terms of political system or danger posed to the region, although US President George W Bush has called Iran part of an axis of evil.

Straw said that no one knew if Iran was developing nuclear weapons, and added that he would urge Iranian President Mohammad Khatami later on Monday to allow more intrusive inspections of Iran's nuclear facilities by the UN's International Atomic Energy Agency.

"What we...are proposing is that they (the Iranian authorities) have to sign up to an additional protocol which provides for more intrusive inspections," Straw said.

"The first consequence if they fail to sign up will be that they are not able to make progress on other issues," including a planned trade agreement between Iran and the European Union, Straw said.

However, when asked if he believed there were no circumstances in which Britain would agree to an attack on Iran, Straw replied, "Yes, and I can conceive of no such circumstances."

Reuters adds: Iran will shortly invite International Atomic Energy Agency Chief Mohammed ElBaradei to Tehran to discuss its nuclear program, the Middle Eastern country's official news agency reported on Monday.

"Iran is going to invite ElBaradei soon to visit Iran to hold talks to remove technical problems," IRNA quoted Iran's Supreme National Security Council Secretary General Hassan Rohani as saying during a meeting with visiting British Foreign Secretary Jack Straw.

Straw was on a visit to Tehran to urge Iran's government to sign up for tougher inspections of its nuclear facilities immediately and unconditionally.

Meanwhile, Iran's clerical leaders on Monday showed no signs of giving into mounting international demands they open up their suspect nuclear programme to tougher UN inspections, despite warnings by visiting British Foreign Secretary Jack Straw of economic consequences.

Straw, in Iran for his fourth visit in just 19 months, told the Islamic republic to "unconditionally" sign an additional protocol to the nuclear Non-Proliferation Treaty (NPT) that would allow the UN's atomic energy body to carry out surprise probes of nuclear facilities.