The African Union (AU) and the United Nations are chairing talks in Libya to seek a blueprint for peace in the western Sudanese region of Darfur.
Millions of people have been displaced by the fighting in Darfur
The AU and the UN have laid out a three-stage "road map" which they hope will lead to peace talks between rebels and Khartoum in the next few months.
A new alliance of five rebel factions has been formed in order to present a united front for negotiations.
The Tripoli talks come as the UN says the situation in Darfur is worsening.
Some 200,000 people have died in Darfur in the past four years and two million have fled their homes, the UN says.
A widely unpopular peace deal signed between the government and one of three rebel groups last year has been largely ineffective and the groups have since splintered into factions.
The new rebel alliance, calling itself the United Front for Liberation and Development, agreed to unite in the Eritrean capital, Asmara, on Saturday.
It includes the Revolutionary Democratic Front Forces, the National Movement for Reform and Development, the Sudan Federal Democratic Alliance and two factions of the Sudan Liberation Army.
The groups said they had joined together to present a united front for peace negotiations with Khartoum and appealed "to all other movements to unify efforts".
But several major rebel groups remain outside the new alliance.
Representatives from the five permanent members of the UN Security Council, the EU, key donor countries and Sudan's neighbours have all been invited to the talks in Tripoli, which will last for two days.
Some rebel representatives are also currently in Libya, but they will not be taking part in the talks.
The AU and the UN have been working hard to streamline competing and often confusing peace initiatives for Darfur.
The Libyan head of AU affairs, Ali Traiki, has said the meeting will seek to lay out a timetable and date for talks between the government in Khartoum and the non-signatory rebel factions.
Meanwhile, UN special envoy for Darfur Jan Eliasson remains reluctant to specify a date for the renewed talks, says the BBC's Rana Jawad in the Libyan capital.
Sunday's talks come just hours after the US special envoy to Sudan, Andrew Natsios, accused the Sudanese government of resuming bombing in northern and western Darfur.
African Union peacekeepers have been unable to end the fighting
Mr Natsios urged Sudan to stop its campaign and respect a 2004 ceasefire.
He also said accused rebel groups of descending into "criminality and warlordism".
Meanwhile, UN reports say security on the ground is also deteriorating, pointing to the rise in attacks on aid workers and AU peacekeepers in Darfur.
The talks in Tripoli come as the UN is examining a revised Security Council draft resolution for a joint AU-UN African peacekeeping operation for Darfur.
The Sudanese government has finally agreed to allow a 20,000-strong hybrid force into the region after months of pressure from the international community.