Syrian peace talks in Switzerland hit a major hurdle on Monday after a UN invite to Iran sparked a boycott call from the opposition, as President Bashar al-Assad ruled out a power-sharing deal.

In an exclusive AFP interview, Assad said there was a "significant" chance he will seek a new term and called for the talks opening Wednesday to focus on what he called his "war against terrorism".

With two days to go to negotiations more than a year in the making, UN chief Ban Ki-moon sparked a furore Sunday by inviting Iran -- a key backer of Assad -- to take up a seat.

Syria's main opposition group promptly said it would shun the talks, due to kick off in Montreux before moving to Geneva.

Louay Safi, spokesman for the Syrian National Coalition, announced on Twitter that the opposition would stay away "unless Ban Ki-moon retracts Iran's invitation".

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.

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

The accord at the so-called Geneva I talks made no mention of Assad's departure, something that the Syrian opposition says is non-negotiable.

To add to the confusion, Russia -- another Assad ally -- said Tehran's absence from the conference would be a "unforgivable mistake."

Moscow along with Washington has been one of the main backers of the new round of talks.

Ban's invitation came after Tehran 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.

Making the announcement after two days of intense talks with Iranian Foreign Minister Mohammad Javad Zarif, Ban said both Zarif and he had agreed "that the goal of the negotiations is to establish, by mutual consent, a transitional governing body with full executive powers."

"I believe strongly that Iran needs to be part of the solution to the Syrian crisis," he said.

But Ban's assurances failed to pacify major Western powers concerned over Iran's deployment of military personnel in Syria and its support for Lebanon's Hezbollah movement, which has sent fighters to back up Assad's troops.

"The United States views the UN secretary-general's invitation to Iran to attend the upcoming Geneva conference as conditioned on Iran's explicit and public support for the full implementation of the Geneva communique," US State Department spokeswoman Jen Psaki said in a statement.

"This is something Iran has never done publicly and something we have long made clear is required."

French Foreign Minister Laurent Fabius said Iran's participation was conditional on clearly accepting the "establishment of a transitional government in Syria with full executive powers."

British Foreign Secretary William Hague also said Iran must publicly express support for a transitional government, saying: "The world will want to hear that."

Source: AFP