The decision to appoint a new envoy to the United Nations comes as nuclear talks between Iran and the United States and other major powers are coming under fire from U.S. congressional critics of Iran, who are pressing for the passage of new sanctions that the White House claims will torpedo the delicate negotiations. 

China gave a low-key welcome to Iran's President Mahmoud Ahmadinejad as he visited the Shanghai Expo on Friday, just days after Beijing voted for a fourth round of United Nations sanctions designed to curb Iran’s nuclear program.