The United Nations says the special envoy to Syria will start in-depth consultations with Syrian, regional and international parties in May to try to relaunch political negotiations aimed at ending the Syrian conflict, which is now in its fifth year.

In late March, Secretary-General Ban Ki-moon ordered envoy Staffan de Mistura to consult widely and "to operationalize and flesh out" elements of the 2012 Geneva roadmap to peace.

U.N. spokesman Stephane Dujarric said Tuesday the list of invitees to the one-on-one discussions has not yet been decided.

Iran wasn't invited to the last negotiations, which collapsed in February 2014.

Asked whether Iran would be consulted this time, Dujarric replied: "I think it is important that all those who have an influence on the current conflict be included in discussions."