BEIRUT - A senior member of Syria's Western-backed political opposition said Iranian participation in diplomatic talks on the conflict this week in Vienna could obstruct a political solution to the four-year-old war.The United States said on Tuesday Iran, the main regional ally of President Bashar al-Assad, would be invited to participate in international talks on Friday to discuss ending the conflict.About a dozen participants are expected including Russia and Saudi Arabia, which back Assad and his opponents, respectively. It is not clear whether any Syrian participants will attend.Syrian National Coalition vice president Hisham Marwa criticised any participation in the talks by Iran."Regarding the issue of Iran's invitation...Iran's presence will complicate the Vienna talks because it will come with a project to preserve Assad," he told Saudi-owned al-Arabiya television in an interview late on Tuesday."The Russian intervention blocks a political solution and Iran's involvement was, and still is, able to obstruct a political solution," he said.Iran said on Friday it was considering whether to attend."Iran's Foreign Minister Mohammad Javad Zarif and his Russian counterpart spoke on the phone yesterday and today about Syria and the coming summit in Vienna...Iran's participation is under discussion," the Fars news agency quoted Foreign Ministry spokeswoman Marzieh Afkham as saying.The Turkey-based Coalition has little influence over rebels fighting to overthrow Assad in a conflict that has been complicated by the successes of rival hardline Islamist groups.Its critics say the exiled group fails to represent the Syrian people and its decisions are dictated by its two main backers - Saudi Arabia and Qatar - which compete for influence.Russia intervened militarily in Syria's civil war at the end of last month in support of Assad, and the Syrian army is also supported by Iranian fighters.Russia and Iran say Assad must be part of any transition and that the Syrian people will decide who governs them. The United States has said it could tolerate Assad during a short transition period, but that he would then have to exit the political stage.