UNITED NATIONS - The United Nations mediator in Syria's conflict on Wednesday proposed inviting warring parties to take part in four UN-led working groups on how to implement a roadmap to peace, since the groups were not ready to hold formal peace talks.Mediator Staffan de Mistura told the UN Security Council that the groups would address safety and protection for all, political and constitutional issues, military and security issues, and public institutions, reconstruction and development.UN Secretary-General Ban Ki-moon said the aim was to set up the working groups as soon as possible. He said he was ready to convene a high-level conference to endorse any framework agreement they reached.The briefing was the culmination of nearly three months of consultations with Syrians and world and regional powers on implementing the Geneva Communique, a roadmap adopted by world powers in June 2012 calling for political transition while leaving the role of Syrian President Bashar Assad unresolved."Sadly there is still no consensus on the way forward on the Communique or yet a formalized negotiation," said de Mistura, whose two predecessors resigned in frustration at the failure to make headway in ending the Syrian war."While common ground exists, the questions over devolution of executive authority to a transitional body, let's be honest with ourselves, remains the most polarized element of the Communique," de Mistura said.He said the working groups could be a step towards a "Syrian-owned framework document" that would provide for a transitional body, procedures for national dialogue, a constitution drafting process and transitional justice issues. Syrian UN Ambassador Bashar Ja'afari said Damascus would study de Mistura's proposals closely before responding.A Syrian government crackdown on a pro-democracy movement in 2011 led to an armed uprising. Radical Islamic State militants have since seized on the chaos to declare a caliphate in territory they have seized in Syria and neighboring Iraq.Western powers have supported non-Islamist rebels in Syria and said there was no room for Assad in a future Syria.Ban said at least a quarter of a million people had been killed during the more than four-year-old civil war.US Secretary of State John Kerry plans to discuss Syria with Russian Foreign Minister Sergei Lavrov in Doha next week. He said they would see what Iran, which supports Assad, is prepared to do.Russia, also a Syrian ally, is trying to bring about rapprochement between Assad's government and regional states hostile to it, including Saudi Arabia and Turkey, to forge an alliance to fight Islamic State.A US-led coalition has been bombing Islamic State targets in Syria since September.