Western countries sought to narrow their differences on Wednesday on how to fund Afghanistan after foreign combat troops leave at the end of 2014, aiming for an agreement next month as more allies prepare to bring their forces back early.

Speaking before a meeting of foreign and defence ministers from the 28-country NATO military alliance, Secretary General Anders Fogh Rasmussen said a summit in Chicago on May 20-21 would hammer out the finances for post-2014.

At the Chicago summit we will get a clear picture of the commitment to financing the Afghan security forces, Rasmussen told reporters. We will discuss that at this meeting.

A timetable agreed between NATO and Afghanistan in 2010 called for the withdrawal of foreign combat troops from Afghanistan by the end of 2014, when Afghan security forces are supposed to have full control.

There are concerns that a rapid handover to Afghan forces will leave the country weak and vulnerable to attacks by the Taliban, similar to this week's assault on Kabul's diplomatic district, and could accelerate a descent into civil war.

Australia joined other powers on Tuesday in seeking an earlier withdrawal when it said it would start pulling out its troops this year. France has said it will withdraw its troops by the end of 2013 and New Zealand may pull out before 2014.

Financing for training and equipping Afghan security forces post-2014 is expected to be substantial, with the country supporting a total security staff of 350,000 personnel, including 195,000 members of the Afghan national army.

Other troops are due to stay to provide training and guidance, another costly enterprise. With Western budgets tight due to an economic downturn, securing financial commitments is likely to prove tough, especially from European states.

Afghan President Hamid Karzai has said he wants at least $2 billion a year from Washington after 2014.

No final decision has been made as regards the size, structure and cost of the Afghan security forces in the longer term, Rasmussen said, although he said a figure of around $4 billion a year had been mentioned.

I would expect NATO allies and ISAF (the International Security Assistance Force) to commit themselves to pay a fair share of the total bill.

A U.S. official said on Tuesday that the United States believed it would be able to announce in Chicago commitments from NATO allies and other partners worth 1 billion euros ($1.31 billion) a year.

Efforts are being made to have non-NATO and non-ISAF countries - such as the Gulf states or perhaps Japan - contribute to the bill, an indication of how hard officials are finding it to drum up Western contributions.

UNSTABLE STATE

This week's meetings, on Wednesday and Thursday, come after renewed violence has further shaken Western public support for Afghan operations.

On Sunday, insurgents attacked the heavily guarded diplomatic district of the Afghan capital, sparking 18 hours of fighting. Thirty-five insurgents were killed by Afghan forces backed by NATO troops. Eleven Afghan troops and four civilians were also killed.

Responding to Australia's move to speed its exit, Rasmussen said that Prime Minister Julia Gillard had made clear Australia would remain committed to Afghanistan after 2014.

He also said Taliban or Taliban-linked attacks were down 9 percent in 2011 from 2010, and that they were down around 10 percent during the first months of this year.

While that may be the case, the country is far from stable, and efforts to build a government that has the people's trust have proven a challenge amid widespread evidence of corruption.

As Western forces prepare to leave, NATO officials are increasingly praising the ability of Afghan security forces, who largely dealt with Sunday's attacks.

We have really now seen in practice the Afghan security forces deal in a very professional way with security challenges, Rasmussen said.

Ministers will also discuss ways to maintain defence capabilities with pressured budgets. On Thursday, Russian Foreign Minister Sergei Lavrov is expected to attend the meeting and discuss NATO's missile defence system - which Russia sees as a threat.

(Additional reporting by Arshad Mohammed and Missy Ryan; editing by Elizabeth Piper)