Veteran diplomat Lakhdar Brahimi will replace Kofi Annan as special envoy to Syria, in what the the French ambassador dubbed "the impossible mission".
UN sources told Reuters that Brahimi will have an altered mandate, for a position he was reluctant to take.
Brahimi has urged the UN Security Council to unite behind him as a condition for him taking over from Annan, who resigned in despair earlier this month citing a lack of support from major powers on the Security Council.
Lakhdar Brahimi will replace Kofi Annan as special envoy to Syria
Syria's President Bashar al-Assad is believed to be agreeable to Brahimi's appointment.
The former Algerian foreign minister, who led the UN Assistance Mission to Afghanistan, Brahimi is believed to have demanded a different title and a tailored mandate as a condition for him taking over from Annan.
Russia and China have used their powers as permanent members of the Security Council to veto three resolutions which threaten sanctions against Assad.
The French UN Ambassador Gerard Araud, currently the Security Council president, said he knew the task was " an impossible mission, so I do understand that people hesitate to take this (on)."
An anonymous diplomatic source told Reuters "Many want him to just step into Kofi's shoes and just go on and either persist with the same failed approach or invent something unlikely.
"Even though he is pressured on all sides to accept, he still has not accepted pending a number of basic conditions," the source said. "His conditions are the minimum for such a mission to go on."
Writing in the Huffington Post, Sami Moubayed, a Visiting Scholar at the Carnegie Middle East Centre in Beirut, said Brahimi would not risk his reputation lightly.
"Brahimi is an Arab Muslim who knows the Middle East inside out; not through the narrow prism of the UN, but from hands-on experience from having lived and worked with Arabs for his entire career.
"His positions brought him often to Damascus, his last visit as part of The Elder's delegation in October 2010. In 1989, he worked with the Syrians on hammering out the Taif Accords to put an end to Lebanon's civil war.
"He knows their fears, concerns and mentality, and has dealt with every Syrian administration from Nour Al Din Al Atasi to Bashar Al Assad.
"Brahimi won't take things at face value, as Annan did. He would know when the Syrians are lying to him about something.
People trust and respect him, not only in Syria but globally."