The NATO Summit, which begins over the weekend in Chicago, will focus on Afghanistan, in particular on the next steps of transition and the post 2014 Afghanistan, a top White House official has said.

This summit is about the next steps in transition, role of international community post 2014, and discussion of, and agreement on the structure and sustainability of the Afghan national forces, the US National Security Advisor Tom Donilon told reporters here during a news conference on Thursday.

"So Chicago is a critical milestone in the next step towards a responsible ending of this war, towards our achieving, very importantly, our goals in this effort in Afghanistan," Donilon said in his briefing on the NATO Summit.

The top Obama aide said that with respect to the next steps in transition, the next steps toward 2014, is that the alliance will decide that in 2013 that the mission will shift for its forces.

"That is that the mission will shift from the ISAF forces, the United States forces, as part of ISAF, being in the combat lead to stepping back and getting into principally a train-and-advise mode, with the Afghans going into the combat lead all over the country," he said.

"That's essential if you think about how you get to the end of 2014 with full Afghan responsibility for their security. You need to start that process. You need to get the Afghans out front with the United States and its allies and partners supporting them moving forward. That's the first element of what will be talked about and decided at Chicago," he said.

The second element would be a discussion of and agreement on the structure and sustainability of the Afghan national forces past 2014.

"That is, what should their size be, what should the mission be, and how will it be paid for? Sustainment, of course, is a euphemism for how will it be paid for going after 2014. And we've made very good progress on this," Donilon said.

Currently the strength of Afghan forces is 330,000, which would surge up to 352,000. "We will then, at some point after 2014, start to go down to a sustainable level -- and we're working through the modeling on that -- of Afghan forces that'll be the level that will be required, as assessed by our military in conjunction with the Afghans, going forward," he said.

Donilon said that the cost of sustaining such a force is estimated to be USD 4 billion a year. The US has been working with its ISAF partners to get multiyear commitments from them.

"We have had over 30 countries make commitments," he said. Some of the countries like Britain (USD 110 million a year), Australia (USD100 million) and Germany (USD 195 million a year) have already announced their commitment. Some other countries are expected to make announcement in this regard during the summit.

"The third thing that'll be discussed at Chicago will be the nature of the presence in Afghanistan after 2014. After the ISAF combat mission ends, what are the plans for NATO? And there'll be a discussion about focusing on a much smaller-size NATO training and assisting and advising mission in Afghanistan," Donilon said.

"The goal is to have an Afghanistan again that has a degree of stability such that forces like al-Qaeda and associated groups cannot have safe haven unimpeded, which could threaten the region and threaten US and other interests in the world,".

He said the other goals would be to have an Afghanistan with a set of security assets that allow it to provide for that modicum of stability and to be able to protect itself against groups like that.

Moreover, the top Obama aide said that the third goal would be to have an Afghan national force of sufficient size and sustainability so that these goals can be achieved.

Donilon said that however, the core goal is the strategic defeat of al-Qaeda so that it no longer presents a threat to the United States, its allies or its other interests.

"And as you know, this has been a central part of the foreign policy of the United States, especially in terms of its focus, since we've come into office. And again, this is a daily effort that we pursue relentlessly against al-Qaeda," he added.