Attendance by Burns will mark a shift in U.S. policy. The Bush administration has long insisted on shunning nuclear talks with Iran until it stops enriching uranium.

• 伯恩斯参加谈判标志着美国政策的转变。布什政府长期以来坚持不和伊朗举行核谈判，直到伊朗停止浓缩铀活动。