NATO foreign ministers were set to try Tuesday to nail down an accord with Kabul on the alliance's new role in Afghanistan as pressing problems in Ukraine, Syria and beyond all call for attention.

The immediate issue is NATO's planned training and advisory mission in Afghanistan after it ends its biggest ever combat operation there next year, clearing the decks for leaders to set a new course for the alliance at a summit in late 2014.

Officials say the focus of the two-day meeting in Brussels is to build on NATO's active military role since the early 1990s, from the Balkans to Afghanistan and Libya, safeguarding gains in inter-operability and capability at a time when defence budgets are under strain.

The aim is a NATO which remains relevant and effective in a changing world where the challenges are as much military as political and economic, threatening to boil over into conflict and social upheaval.

    "We have got to ensure that we sustain NATO's military edge," a senior US official said

"In the context of extreme budgetary constraints ... it is incumbent on us all to do more with (the money) that we have''.

The 28 allies, plus NATO's partners and sometimes adversaries such as Russia, will review issues such as how to destroy Syria's chemical weapons arsenal.

Missile defence, a hugely sensitive issue for Moscow, is on the agenda given US and European concerns of a threat from Iran despite the recent signing of an initial deal on its contested nuclear programme.

Relations with Ukraine and Georgia provide another difficult issue for Russian Foreign Minister Sergei Lavrov and his US counterpart Secretary of State John Kerry as NATO seeks to maintain and boost ties with former Soviet states.

Continued and growing protests in Ukraine after the government ditched a planned association accord with the EU are likely to test ties.

Following the NATO meeting, Kerry will travel to Moldova which did sign up with Brussels last week despite intense Russian pressure not to.

"''We are making this brief stop to demonstrate US support for the important choice that Moldova made," the US official said

On Afghanistan, the sticking point is President Hamid Karzai who is refusing to sign a Bilateral Security Agreement with Washington until after April's presidential elections, when he stands down.

The BSA sets the legal and operational framework for the up to 12,000 troops, mostly American, who will train and assist Afghan armed forces post-2014.

Without a BSA, and a parallel accord for the allies, there is "no post-2014 mission," a senior NATO official said Monday.

In 2011, the US government took the 'zero option' of complete withdrawal from Iraq when it could not get a troop status deal.

Diplomatic sources stressed that military and political realities allow now only limited time for the post-2014 Afghan mission to be finalised.

'' You cannot wait until mid-2014 and expect the United States to (then) write a big cheque" one senior NATO diplomat said, adding: "This has been made very clear''.

Other NATO officials warned that future aid could be put at risk without a deal as donors want the reassurance of a troop status accord and continued alliance presence..

One official put aid for the Afghan armed forces at $4.1 billion annually -- of which Kabul at best could only raise $500 million -- and $4.0 billion for development.

If there is no post-2014 mission, funding "in theory could continue to be forthcoming ... but in practice there must be a question whether donors would have the confidence to contribute," the official said.

Karzai on Sunday accused Washington of halting essential supplies to some army and police units in an effort to force him to sign, a charge rejected.

About 75,000 NATO combat troops are still deployed in Afghanistan, the majority of them American, and are being steadily drawn down.