South Sudan says it sent forces into Heglig after Sudan attacked its territory
Nato foreign and defence ministers are set to meet in Brussels to discuss how to fund security forces in Afghanistan once international troops leave.
They insist there will be no change in strategy despite Sunday's co-ordinated attacks by the Taliban.
Nato has agreed to start handing over security to Afghan forces, leaving them in full control from the end of 2014.
The US is expected to ask international partners to provide up to a quarter of the costs of backing Afghan forces.
Washington is looking for other countries to pay up to $1 billion (£630 million) a year, while it provides around $3 billion a year in support.
The meeting is preparing for a summit of Nato heads of state being held in Chicago next month, where a final commitment on funding Afghan security is expected to be announced.
The BBC's defence correspondent Caroline Wyatt says one of Nato's toughest jobs now will be holding the military coalition in Afghanistan together as the alliance's combat mission draws to a close.
On Tuesday Australia announced that it was bringing home most of its troops almost a year earlier than expected.
Other nations in the coalition, including Canada, the Netherlands and France have already pulled their forces out of combat or speeded up their withdrawal.
Afghans 'can cope'
The Afghan National army will number around 350,000 soldiers in the next few months, with the US providing most of the training and logistical support.
A US plan predicts the Afghan army will be cut to around 230,000 in few years time. The planned reduction has worried some security experts in Afghanistan.
Co-ordinated gun and rocket attacks on seven sites in the Afghan capital Kabul on Sunday saw two Afghan soldiers killed and 17 militants.
Nato officials have acknowledged that the attacks helped highlight the challenges ahead, but the BBC's Caroline Wyatt says they are keen to stress their view that the attacks show that the Afghan army can cope in providing security.
Nato spokeswoman Oana Lungescu pointed out that Afghan forces dealt with the situation and defeated the attacks, largely on their own.
"Clearly, we still face security challenges," she said. "This was not the first such attack, and I don't expect it will be the last. But such attacks don't change the transition strategy."
Nato ministers will also be discussing how to improve the sharing of equipment among European members in an initiative known as "smart defence", as they try to cope with falling defence budgets.
During the two-day meeting they will also discuss the next stages of Nato's plans for a missile defence system across Europe.
High-profile attacks on Kabul
- 15 April 2012: Seven sites including parliament, Nato HQ and foreign embassies attacked
- 13 September 2011: Gunman seize unfinished high-rise to fire on Nato HQ and US embassy
- 19 August 2011: Gunman storm British Council HQ, killing 12 people
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