The United Nations and Arab League special envoy to Syria, Lakhdar Brahimi, has announced that he is stepping down from his role.
Mr Brahimi took over the role in 2012 from former UN chief Kofi Annan.
Both have failed to find a peaceful solution to a three-year conflict that has left some 150,000 people dead.
Meanwhile the French foreign minister claimed he had "credible" evidence that the Syrian government had used chemical weapons dozens of times since 2013.
"We have credible witnesses for the use, at least 14 uses... of chemical agents since October 2013," said Laurent Fabius during a visit to Washington.
He said the attacks were "small-scale" and included the use of chlorine gas.
Last summer, under the threat of military action, Syrian President Bashar al-Assad agreed to destroy his chemical weapons stockpile.
UN Secretary General Ban Ki-moon praised the veteran diplomat but admitted that the UN had "not been able to make any progress" in Syria.
Mr Ban told a news conference in New York that Mr Brahimi's term would come to an end on 31 May.
He said that Mr Brahimi had faced "almost impossible odds" because of an international community that was "hopelessly divided".
Mr Ban gave no indication whether or not there would be a successor.
The Algerian diplomat, 80, said that he would brief the UN Security Council on his decision later on Tuesday.
Arab newspaper reports suggested that Mr Brahimi has been considering the move since President Assad announced new presidential elections earlier this month.
Mr Brahimi had been pushing for a transitional government made up of government and opposition figures.
He is one of the world's most experienced international diplomats, having previously served as UN envoy to South Africa and Afghanistan.
Mr Brahimi chaired two round of peace talks between the Syrian government and the opposition movement in Geneva, but the negotiations ended in deadlock.
In the latest round in February, both sides agreed to some limited local ceasefires to allow for humanitarian assistance, but fighting still continues in large parts of Syria.
In addition to those killed, millions of Syrians have been forced to flee their homes since the uprising against President Assad began in March 2011.
Large parts of Syria's main cities, particularly Aleppo and Homs, have also been destroyed.