"The ongoing nuclear negotiations between Iran and the Sextet of powers are at a critical stage," Mann said in statement released on Thursday.
He noted that the two sides have been trying hard to make progress, and said, "We remain fully engaged to attain a comprehensive solution by November 24 deadline."
He underlined that the diplomatic efforts to find a resolution to the Iran's nuclear standoff with the West are now in a "critical phase".
Iran and the six major world powers ended their 8th round of nuclear negotiations in Vienna on Thursday.
Iranian Foreign Minister Mohammad Javad Zarif and EU Foreign Policy Chief Catherine Ashton presided over the 8th round of the talks between Tehran and the G5+1 in the Austrian capital.
The 7th round of talks between Iran and the Group 5+1 was held in New York on the sidelines of the UN General Assembly meeting last month.
The two sides held six rounds of negotiations in Vienna to reach a comprehensive deal after they inked an interim agreement in Geneva on November 24.
The Geneva agreement took effect on January 20 and expired six months later on July 20. In July, Tehran and the six countries agreed to extend negotiations until November 24 after they failed to reach an agreement on a number of key issues.
