The United Nations has taken control of the peacekeeping mission for Darfur in Sudan after months of negotiations but it remains seriously under strength.
The African Union soldiers now work for the UN force
The force, Unamid, which replaces the African Union mission, is supposed to become the world's largest peacekeeping force with 26,000 troops.
The commander of the 7,000-strong AU force swapped his green beret for the blue one of the UN at a ceremony.
Sudan has been accused of blocking the full deployment of the force.
It insists that only African and Muslim troops can take part, while contributing countries have been slow to offer the helicopters seen as vital to the force's ability to move around and quell the violence.
At least 200,000 people have died in the five-year conflict, which has led some 2.5 million people to flee their homes.
Unamid head Rodolphe Adada called for more troops to be sent "as quickly as possible."
"We are determined to deploy the most robust force possible so that it can carry out effectively the difficult mandate the Security Council has entrusted to it," he said as the UN flag was hoisted in the North Darfur capital, el-Fasher.
An extra 2,000 personnel have been sent to Darfur and African Union spokesman Noureddine Mezni said more would arrived by mid-January.
He said it would take "months" to reach the target figure of 26,000 and called for helicopters to be sent urgently.
"In an area like Darfur, the size of France, we cannot do the job properly without these things. We appeal to the international community and all those able to provide us with these things to do so as soon as possible," the AP news agency quotes him as saying.
The transfer of control comes as Sudan accuses Chadian forces and Darfur rebels of launching a new offensive in the region.
A number of towns and villages in western Darfur have been under attack from the combined force since Friday.
Chad denies its troops have crossed the border but diplomatic sources, who have asked not to be named, have said a number of Sudanese towns and villages in the area of el-Geneina are under threat.
The Sudanese government has complained to the UN Security Council.
Chad has said its ground and air forces have attacked rebel targets along the border, but have not crossed into Sudan.