NATO allies closed ranks Wednesday to ensure an orderly troop withdrawal from Afghanistan over the next two years as Australia became the latest nation to map out its pullout from the war.

Foreign and defence ministers gathered in Brussels for rare joint talks to discuss their Afghan strategy and debate the future size and funding of Afghan security forces after foreign combat troops have left at the end of 2014.

The talks, aimed at preparing a NATO summit in Chicago on May 20-21, follows a Taliban onslaught in Kabul at the weekend that underscored the insurgency's resiliency even as officials insist that militants are on the backfoot.

Afghan security forces are gradually taking over control of security in their country, with the goal of being in the lead nationwide next year and paving the way for most of the 130,000 foreign troops to leave by the end of 2014.

But Australian Prime Minister Julia Gillard said on Tuesday that her country would bring its troops home a year earlier than planned, with most soldiers withdrawn in 2013 after significant security gains over the past 18 months.

NATO Secretary General Anders Fogh Rasmussen insisted that Australia's announcement was "fully within the framework" of a transition roadmap agreed by NATO and its partners in Afghanistan.

"All 50 allies and partners within the ISAF coalition have committed themselves to the basic principle of, 'in together, out together.' And I know that the Australians are committed to that principle as well," Rasmussen said.

But German Defence Minister Thomas de Maiziere said he was "surprised" by Gillard's announcement, saying that his Australian counterpart had declared "something different" during alliance talks in February.

Another major contributor in Afghanistan, France, may also review its own pullout.

France's right-wing President Nicolas Sarkozy announced earlier this year that French troops would switch from a combat to a support role in 2013.

Sarkozy is facing a stiff challenge from Socialist candidate Francois Hollande who wants to bring troops home by the end of this year. Hollande is leading polls to defeat Sarkozy in the two-round election taking place on Sunday and May 6.

As allies chart the withdrawal of combat troops, the 50-nation coalition in Afghanistan is debating the price tag to sustain Afghan security forces after 2014, estimated at $4.1 billion a year.

NATO expects Afghan security forces to grow to 352,000 soldiers and policemen this year but the future size is also under discussion, with a US plan foreseeing a reduction of the Afghan forces to 228,500 in 2017.

Rasmussen stressed that no decision would be taken on the size and funding of Afghan forces during the two days of talks in Brussels.

Afghan President Hamid Karzai on Tuesday said he wanted the United States to commit on paper that it would provide "at least $2 billion" a year after US combat troops withdraw in 2014.

NATO's role after 2014 is also under discussion, with Rasmussen saying that the alliance would continue to train, assist and advise Afghan security forces but would no longer go into combat.

US Secretary of State Hillary Clinton and Defence Secretary Leon Panetta were to discuss other complex issues with their counterparts, including plans to share military capabilities amid budget cuts and thorny ties with Russia.

Clinton and her 27 NATO allies will hold talks with Russian Foreign Minister Sergei Lavrov on Thursday but no progress is expected on negotiations to ease Moscow's concerns about a US-led missile shield in Europe.

NATO had hoped to invite Russia to its Chicago summit but Russian president-elect Vladimir Putin is not expected to attend. Alliance officials say a scheduling conflict prevents him from showing up.