In a joint statement after talks in Vienna, Austria, on Friday, participants also called for renewals of stalled UN-brokered talks between the government and opposition and fresh elections.
They said "substantial differences remain", though they agreed it was "imperative to accelerate all diplomatic efforts to end the war", Reuters reported.
The participants' statement said they were asking the United Nations to bring together representatives of the Syrian government and opposition to launch "a political process leading to credible, inclusive, non-sectarian governance, followed by a new constitution and elections".