Six Revolutionary Guards commanders killed.
Total 28 die, 28 wounded at meeting of tribal chiefs.
Revolutionary Guards blame foreign elements linked to U.S.
State TV says Sunni rebel group claims responsibility.
A suicide bomber killed six senior Revolutionary Guards commanders and 23 other people in one of the boldest attacks on Iran's most powerful military institution. The attack, in the country's turbulent southeast, comes as Iranian officials are due to meet their Western counterparts in a sensitive second round of negotiations in Vienna on Monday intended to resolve a standoff with the West about Iran's nuclear ambitions.State media said a local rebel group called Jundollah (God's soldiers) claimed responsibility for the attack, the worst on the elite Revolutionary Guards in recent years, which also injured another 28 people at a meeting of tribal chiefs."Rigi's terrorist group has claimed responsibility for the attack," it said, referring to Abdolmalek Rigi, leader of Jundollah (God's soldiers) which is linked by some analysts to the Taliban in neighbouring Pakistan.But the Guards themselves accused "foreign elements" linked to the United States of involvement. Tehran accuses the United States of backing Jundollah to create instability in the country, a charge that Washington denies.State television also pointed the finger at Britain, another traditional foe of Iran.The Revolutionary Guards is an elite force seen as fiercely loyal to Iran's Supreme Leader, Ayatollah Ali Khamenei. Its power and resources have increased in recent years. It handles security in sensitive border areas.