Khartoum, Sudan (CNN) -- The United Nations-African Union joint special representative to Darfur, Ibrahim Gambari, has called on leaders of Darfur rebels to join the peace process.
"I reiterate my call to the Justice and Equality Movement, the Sudanese Liberation Army-Minni Minnawi (faction) and the Sudanese Liberation Army-Abdul Wahid (faction) to join the peace process and contribute to the building of a better tomorrow for all Darfuris," he said at a news conference in Khartoum on Wednesday.
"This is not the time to posture. Too many have suffered and too much is at stake. Entire communities are looking at your leadership to take Darfur forward," he added.
The government of Sudan and one Darfur rebel group, the Liberation and Justice Movement, signed a peace agreement in Doha, Qatar, last July under the sponsorship of the United Nations, the African Union and the Arab League.
But several other rebels groups have not signed the Doha Document for Peace in Darfur.
Gambari, however, remains hopeful.
The Justice and Equality Movement "has not abandoned the Doha process," he said. "They have been around for the negotiations (and) I have been in contact with (them) to see how we can jump-start the negotiations between (them) and the (Sudanese) government."
Justice and Equality is considered by most analysts to be the most powerful of the armed groups, but has received a severe blow with the fall of the regime in Libya of Moammar Gadhafi, who supported the group. Unconfirmed reports say its leader, Khalil Ibrahim is now in Darfur after escaping fighting in Libya.
As for Minni Minnawi's faction of the Sudanese Liberation Army, Gambari noted Minnawi was part of a previous peace agreement, and "we want to engage with him and his movement to reconsider going back to the political process."
Minnawi signed an agreement with the Sudanese government in 2006 in Abuja, Nigeria, and became a "senior assistant" to President Omar al-Bashir. Late in 2010, however, he abandoned the agreement and left Khartoum.
"But the biggest nut to crack, it seems is to me, is Abdul Wahid (Nur)," leader of another faction of the Sudanese Liberation Army, "because he has never joined any peace process at all, has never signed any agreement of any kind," Gambari said.
"I think the international community is getting a bit frustrated ... (and) is considering ways to address that specific movement to persuade it in a very targeted manner to join the peace process."
Nur was based in Paris, but recent unconfirmed reports say he is now in Darfur. Nur's power base is believed to be among the displaced populations in camps in Darfur.
War in Darfur broke out in 2003 when non-Arab rebel groups took up arms against the central government in Khartoum, accusing it of discrimination and neglect.
The United Nations says 300,000 have died because of violence, disease and malnutrition. The Sudanese government says 10,000 have died.
A warrant has been issued against President al-Bashir by the International Criminal Court for war crimes, crimes against humanity and genocide in Darfur.
Humanitarian conditions have been improving in Darfur, Gambari said.
"Our figures have shown that the number of armed attacks in all three Darfur states has fallen by as much as 705 over the past three years, which has resulted in more displaced people returning to their homes," he said.
Although 2.7 million people "were displaced at the height of the conflict," he said, "the estimate now is 1.7 million. Frankly, that is a huge change."
But Gambari also cautioned about being overly optimistic.
"The war is not over. We still have clashes in some parts of Darfur," he said. "The challenge remains; we don't have an all-inclusive peace agreement as we speak.
"We should do everything to avoid more wars and move in the direction of peace."