TEHERAN (AFP) - Iran will attend Syrian peace talks this week in Switzerland but without preconditions, Foreign Ministry spokesman Marzieh Afkham told the official Irna news agency. "Based on the official invitation, Iran will participate in this conference without any preconditions," Ms Afkham was quoted as saying.

United Nations Secretary General Ban Ki Moon sparked a furore on Sunday by inviting Iran to the so-called Geneva II peace talks due to open on Wednesday.

But a spokesman has said that Mr Ban was "dismayed" by Iran's refusal to back the Syrian transitional government, and was "urgently considering his options" on the Syria peace talks.

Mr Ban's invitation came after Teheran vowed to play a "very positive and constructive role" in efforts to end Syria's worsening three-year civil war that the UN says has claimed well over 100,000 lives.

Iran is a key backer of Syrian President Bashar al-Assad.

Washington, London and Paris reacted immediately, saying Iran would have to clearly and publicly support the idea of a Syrian transitional government if it wanted to attend.

The Syrian opposition, meanwhile, threatened to pull out of the conference, leading Ban to announce that "intensive and urgent" talks were being held to resolve the crisis.

Western powers have so far opposed Iran's presence on the grounds that Teheran had not accepted an initial communique adopted by major powers in Geneva in June 2012 calling for the creation of an interim government.

Iran has strongly resisted pressure to accept the communique.

Mr Ali Akbar Velayati, adviser to Iran's supreme leader Ayatollah Ali Khamenei, told Irna that if the invitation to Iran is "based on accepting the Geneva I statement, it means a precondition and Iran will not accept it".

Iran "in no way accepts the Geneva I statement and if the Geneva II conference is based on legitimising the accords of Geneva I, Iran will not view it as a legitimate conference", he said.

Parliament Speaker Ali Larijani explained his country's resistance.

"We were not present at Geneva I, therefore by no means do we accept the Geneva I statement," he told Iran's ISNA news agency.

"A transitional government has two problems: first, it deprives the Syrian people from the right to choose, second, it means letting Syria fall into the hands of terrorists."

If Teheran does attend, there will be 40 countries and a group of regional bodies at the opening meeting, the most intensive diplomatic effort yet to end the war that has caused more than 2.3 million people to flee Syria with some 6.5 million internally displaced.