"Within three months we will not be able to pay the wages of our troops who are on the ground there," an AU official said.
"Everyone knows this mission is important and we think the international community will support us, but they need to do it soon because the money is fast running out," he added.
The 53-nation bloc, which has more than 5,000 troops in Darfur, also believes the international community has not pledged enough cash to finance the US $252 million-a-year mission.
While the international community had provided aircrafts, transport for troops, accommodation and military hardware, only a fraction of the cash needed to actually finance the mission had been offered, the official, who preferred anonymity, said.
Only $79 million had so far been pledged for the peacekeeping operation by the international community - leaving a shortfall of $173 million.
"The international community, UN, European Union and NATO can't ask us to increase our force in Darfur and then not come up with the money," the official added.
Analysts have credited the AU Mission in Sudan (AMIS) with helping calm the situation in some areas of strife-torn Darfur, allowing for the delivery of humanitarian assistance to the region's war-affected populations.
"The AU force has helped to establish more stability. They have done an admirable job, highly professional, with much dedication," Jan Pronk, the Special Representative of the UN Secretary General for Sudan, told the UN Security Council in July.
With more than 5,000 personnel on the ground, the mission expects to increase its personnel to 7,500 by the end of September.
The head of AMIS, Ambassador Baba Gana Kingibe, told IRIN recently that the increase was expected to be completed on schedule.
"We hope that by the end of September, we will cover - more or less - the hot areas, and therefore there should be a general level of calm in Darfur," Kingibe said.
On Tuesday, US Congressman Chris Smith, on the eve of a three-day trip to Darfur said: "There are shortfalls that need to be addressed."
Speaking in the Ethiopian capital, Addis Ababa, after talks with AU officials, he added: "We were told they were $173 million short and would only be able to carry on the programme of deployment for three months, and it is mostly in the area of medicine, but also as to whether the troops will get paid.
"By raising that flag now all of us can ensure that donor countries do their part," Smith said.
In July, Refugees International (RI) called for more support for an expanded, better-equipped and better-trained AMIS to stop violence against women in Darfur.
According to RI, wherever AMIS forces were present, violence had diminished. However, the force still had inadequate numbers and an insufficiently robust mandate to satisfactorily protect the women in Darfur.
"A recent International Crisis Group report argued that 12-16,000 troops are urgently needed in Darfur to provide adequate security," RI noted. "The mandate, however, is perhaps more important. A clear and strong mandate to protect civilians is critical if AMIS is to be able to prevent Darfurian women from being attacked and raped."
The war in Darfur began in February 2003 and pits Sudanese government troops and militias allegedly allied to the government, against rebels fighting to end what they describe as the marginalisation of and discrimination against the region's inhabitants by the state.
The UN estimates that over a third of Darfur's total population -more than 2.5 million people, including nearly 1.9 million internally displaced persons - have been affected by the conflict.