NATO defence ministers on Wednesday focus on planning for Afghanistan after combat forces are withdrawn in 2014, knowing that the transition to full control by Kabul is fraught with danger and problems.

They will also have to review progress in Kosovo, where a NATO-led peacekeeping effort is now in its 13th year, with little sign that the dispute over the country's self-proclaimed independence from Serbia can be resolved.

Afghanistan, however, remains the key issue, with hundreds of billions spent on a war the public has grown disenchanted with, while the problems of withdrawal and what happens afterwards press increasingly for answers.

NATO head Anders Fogh Rasmussen said Tuesday that, despite setbacks in Afghanistan, "the goal, the strategy and the timeline remain unchanged," with the alliance beginning work on a training and assistance mission from 2015.

Rasmussen insisted that the Taliban would not succeed, but one diplomatic source said 'insider attacks' -- where renegade Afghan soldiers killed their NATO comrades -- were a major concern.

NATO members have "concerns about the safety of their own soldiers but also about the potential impact on the overall effort," the source said.

More than 50 NATO soldiers have died in such attacks this year, leading to the temporary suspension of joint operations with Afghan troops, alliance officials said earlier this week.

The Taliban had jumped on the attacks "as something with strategic potential. This is not something that is yet a strategic threat, it has the potential to be that," said the diplomatic source, who asked not to be identified.

"More important right now is how we respond to it, what measures are taken."

The challenges facing NATO are daunting, with the diplomatic source highlighting the slow progress made in developing local governance in Afghanistan, a key step in winning hearts and minds.

Meanwhile, the Taliban will retreat to remote Pakistan areas, not fully controlled by the government, in the winter months to rearm and build supplies, making the insurgent problem more difficult still to control, the source added.

The other main theme of the meeting at NATO headquarters on the outskirts of Brussels is how to address the tricky problem of military spending at a time when governments are under pressure on all sides to cut budgets.

"This period of economic austerity poses a challenge to defence budgets but it also opens an opportunity for strength and cooperation and new ways to provide security," Rasmussen said Tuesday.

NATO adopted a 'Smart Defence' policy at a May summit in Chicago, aiming to promote increased cooperation and burden sharing to offset smaller budgets.

The United States accounts for the bulk of NATO defence spending and its portion has increased markedly in the last decade, a sore point for Washington in trying to get its allies to do more.

"Let me be clear: we need smart spending and, even more, we need sufficient spending," Rasmussen said.