"We have a shortfall of around 200 million US dollars (167 million euros) as of today," said Said Djinnit, the commissioner of the AU Peace and Security Council under whose auspices the Darfur mission, known as AMIS, falls.
"It is a source of concern," he told reporters at a news conference at AU headquarters in Addis Ababa. "Our partners should realize we have a financial gap."
Djinnit's comments came as the first group of additional AU troops -- a 680-strong Nigerian batallion -- prepares to deploy to Darfur on Saturday to reinforce the some 3,200 men already on the ground.
The dispatch of the new troops marks the start of the expansion of the mission which was approved by the African Union in March and foresees the the force growing to 7,700 by the end of September.
The cash-strapped pan-African body has said it needs 460 million dollars (365 million euros) to pay for the expanded mission and in May international donors, including NATO and the European Union, pledged nearly 300 million dollars to meet that goal.
Djiniit said the African Union was hopeful that a new contribution by Canada, the largest single contributor at the May donors conference, and an EU peace fund of some 250 million euros (300 million dollars) would help make up the current deficit and stressed that the expansion should go ahead as planned.
"By the end of September, we will have all the troops on the ground," he said.
The international community has for some time been urging the African Union to take a more active and expansive role in Darfur where more than two years of war have killed between 180,000 and 300,000 people.
The conflict began in February 2003 pitting the Arab-led regime in Khartoum and pro-government militias against largely black African rebel groups and in addition to the deaths has left 2.4 million people homeless.
The AU mission is monitoring a shaky April 2004 ceasefire between Khartoum, government-backed militias and the two Darfur rebel groups, the Sudan Liberation Movement (SLM) and the Justice and Equality Movement (JEM).
Earlier this week, Khartoum and the rebels reached framework peace deal at AU-mediated talks in Nigeria in which Khartoum agreed to guarantee traditional tribal ownership rights over land in Darfur and grant the region autonomy under a reformed federal constitution.
The declaration of principles is to form the basis for future political dialogue.