As the sun rose over South Africa's dry and sparsely populated Karoo west of Johannesburg, more than 5,000 multinational troops launched a mock attack heralding a long-awaited African Union strike force for the troubled continent.

Men in fatigues peered through binoculars and crouched over maps, coordinating the movements of soldiers from more than a dozen African countries taking part this week in the first field exercise of the African Standby Force (ASF).

The script called for rapid deployment in response to reports of genocide in a fake country called Carana -- a war game with a bitter touch of reality on a continent that has suffered the bloodshed of Rwanda and Darfur.

First proposed in 1997, the ASF aims to have forces from one of the continent's five regional economic blocs on standby at any time, ready to respond swiftly to crises across Africa, with an overall force size of 25,000.

"Given our experiences, specifically in response to conflict in the past, the AU felt the international community very slow to respond," the African Union's head of peace support operations, Sivuyile Bam, told AFP.

"It takes time between the mandate being passed and the forces arriving on the ground -- the rule of thumb is usually nine months.

"The types of conflicts we are dealing with simply do not allow for the luxury of time."

The ASF aims to be able to move in and take action within 14 days of being mandated by African Union headquarters in Ethiopia, and it was that capacity that was being tested on a Karoo farm six hours west of Johannesburg.

- Dependent on aid -

But even as it strains for institutional independence, the AU remains heavily dependent on foreign donors -- including for part of the estimated $15 million cost (13.6 million euros) of the war games from October 19 to November 7.

"The current reality is that AU (peacekeeping) operations are funded above 90 percent by partners including the United Nations, the European Union, the US and the UK," said Bam.

The AU has estimated it will cost $1 billion for the standby force to be fully operational, and has proposed a model that would see it raise 25 percent of the funds of an operation and then rely on the UN to fill the gap.

But for the standby force to be truly "on standby", the funding behind it needs to be more predictable, said Bam.

The troops' ability to respond rapidly is also constrained by the continent's insufficient airlift capabilities, said defence analyst John Stupart.

"There are significant challenges blocking getting troops moved very quickly. The AU states simply don't have the planes to transport people and equipment anywhere very quickly," he told AFP.

"I think the general assumption has been that the ASF is a good idea, but I don't think there's been concerted questioning about whether it really is the right or realistic mechanism.

"It's been modelled as a military module that you plug into a crisis, but Africa is a large place. Its issues are complicated and different.

"The crisis in Somalia is different to Nigeria's fight against Boko Haram, which is different to the ongoing problems with rebel groups in North Africa, which is different to Lesotho's political crisis."

- Problems remain -

The establishment of the force has been delayed for nearly a decade, its deadline pushed first from 2008 to 2010, then 2013, and eventually to December 2015.

The war games in South Africa are designed to see whether the ASF is finally operational, with a report set to be presented to the continent's defence and security ministers at the next AU summit in Addis Ababa in January.

But it's not just the AU watching.

"The EU also needs to decide whether this project is still worth funding," said Stupart.

On the ground, the ASF's proponents admit it's unlikely to tick all the boxes by December.

Commanders at the training exercise told AFP that militarily the troops were ready, but said the legal loopholes permitting their deployment still needed to be closed.

"If you don't have a Memorandum of Understanding between the AU and the Southern African Development Community (SADC) or the Economic Community of Central African States (ECCAS) to move troops, that will become an obstacle," said Zimbabwe National Army Major-General Trust Mugoba.

The key, said Bam, was greater buy-in from the continent.

"If the member states are not willing to step forward and provide the capabilities we need, then we do not have a force."