"Having seen that it is not help that the sides need, but that they cannot agree in the first place, Ms. Tagliavini decided that her future work will not get the peace process moving," the source told RIA Novosti.
The working groups in trilateral talks on the Ukrainian crisis began earlier this week in the Belarusian capital of Minsk. The source said some of their participants "extremely upset" Tagliavini with behavior "that could at times be interpreted as rude."
"The last meetings of the Contact Group and its subgroups demonstrated that the sides' negotiability has virtually gone down to zero."
Vladislav Deinego also said the Swiss diplomat's move was not likely linked to the latest round of talks in Minsk.
Tagliavini, born in Basel in 1950, began her diplomatic career in 1982. She served as the UN Observer Mission in Georgia's (UNOMIG) deputy leader in the late 1990s, then headed it from 2002 to 2006. The New Journal of Zurich daily labeled Tagliavini as "the outstanding diplomat of Switzerland" in 2003.