The United Nations said Friday it would  host separate talks with rival sides in Syria from May 4 in a bid to kickstart stalled negotiations to end the four-year conflict.

"The Geneva consultation invitations have gone out a couple of days ago," UN spokesman Ahmad Fawzi told reporters, saying the talks would last between four and six weeks and would be at "mostly ambassadorial level and expert level".

The talks, to be hosted by UN peace envoy for Syria Staffan de Mistura, will take "stock of where we are now, almost three years after the Geneva communique was adopted and ratified", he said.

Fawzi was referring to the road map allowing for a political transition in Syria, agreed during a first round of Geneva negotiations hosted by one of de Mistura's predecessors Kofi Annan in June 2012.

De Mistura, a Swedish-Italian career diplomat who was appointed UN envoy for Syria last July, is set to address the Security Council in New York later Friday.

It remains unclear whether he will have more success than his predecessors Annan and Lakhdar Brahimi in ending the conflict that has killed more than 220,000 people since March 2011.

In January, de Mistura said conditions were not yet right to try to launch more talks after the two first rounds of negotiations in Switzerland failed.

The second Geneva conference, mediated by Brahimi in early 2014, failed to make any significant headway in ending the war.