Sudan:Janjaweed militias and Sudanese armed forces are massing close to rebel strongholds in northern Darfur, according to human rights campaigners.
Their presence raises the spectre of fresh attacks and the resumption of all-out war during the run-up to peace talks planned to begin at the end of the month.
At the same time, key rebel leaders say they will boycott the talks if Khartoum continues its aggressive moves.
Aid agencies say the fresh violence puts more civilians out of range of help.
In an assessment released yesterday, Amnesty International warned that the Sudanese military was strengthening its hold on at least six towns in northern Darfur, including Tine, Kornoy, Um Baru and Kutum.
It called for the rapid deployment of a 26,000-strong joint African Union and United Nations peacekeeping force.
"The gathering of forces in the north, the burning of Haskanita last week, and the attack on Muhajeriya show the vital importance of ensuring that UNAMID is deployed as soon as possible and has the resources available to protect civilians," said Tawanda Hondora, Amnesty International's Africa deputy programme director.
"There is no time for delays. All signs are there that the people living in the northern area are at great risk, and extra personnel and helicopters under military command must be urgently deployed to the area as soon as possible."
More than 200,000 people have died in four years of fighting, which has also forced two million people to flee their homes.
Earlier this year the Khartoum government agreed to the deployment of the joint international force and to attend peace talks hosted by Libya.
But the past weeks have seen a resumption of violence after a break for the rains. Ten African Union peacekeepers were killed in a raid on their base in the town of Haskanita at the end of September.
So far, the blame has been pinned on breakaway rebel groups, although no one has claimed responsibility. The government responded with bombing raids against rebel targets and burned Haskanita to the ground.
Its Janjaweed allies have also launched an attack on Muhajeriya in the past days in which about 45 people are believed to have been killed.
The town is held by a branch of the Sudan Liberation Army (SLA) allied to the government after signing a peace deal with Khartoum last year.
Its military leader said that deal was now under threat. "From now on, our movement will not stand by and do nothing in the face of such attacks," said Arku Suleiman Dhahia, commander-in-chief of the SLA.
The violent chaos is putting increasing strain on humanitarian workers. The medical charity Médecins sans Frontières yesterday said it had evacuated its team of 16 staff from Muhajeriya, where they operated the only hospital.
Meanwhile, rebels said the violence had not created an environment conducive to negotiation.
"The mediation is unprepared, the methodology is unclear, the participation is unclear, and until this is clear, [we] cannot say if we are going to attend," said Abdel Aziz el-Nur Ashr, a senior Justice and Equality Movement official.
Abdel Wahed Mohamed el-Nur, whose SLA faction refused to sign up to last year's peace deal, said he wanted a strong UN force on the ground before this year's talks.
And even the one rebel faction that had said it would go to Libya is now preparing the ground for a U-turn.
If the offensives continue, the SLM-Unity will "be forced to renounce its engagements toward a ceasefire and work to spread war to other parts of Sudan," said Mahjoub Hussein, a spokesman for the faction.