NATO allies sought Wednesday to ensure a smooth withdrawal from Afghanistan and reassure Kabul that the West would financially back Afghan forces once foreign combat troops are gone in 2014.

US Secretary of State Hillary Clinton and Defence Secretary Leon Panetta met with their European and Canadian counterparts as the alliance fine-tunes the final phase of a pullout from the increasingly unpopular war.

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

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

NATO ministers discussed the future size and funding needed to sustain Afghan security forces after 2014, estimated at $4.1 billion a year, and ensure they have the capacity to fight off Taliban insurgents.

"We cannot and we will not abandon Afghanistan," Panetta said, vowing to keep an "enduring presence" after 2014 to train, assist and advise Afghan security forces.

"The key to our enduring partnership is continued international support," he told a news conference alongside Clinton. "We cannot shortchange the security that must be provided by the Afghan forces now and in the future."

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

Washington wants NATO allies and the international community to foot about half the bill.

British Defence Secretary Philip Hammond announced that London would contribute 70 million pounds a year ($112 million), arguing that it was cheaper to fund Afghan forces than deploy British troops.

"It's a fair sum but it's very considerably less than we're spending at the moment," he told reporters.

NATO expects Afghan forces to grow to 352,000 soldiers and police this year but the United States has proposed to reduce Afghan forces to 228,500 in 2017.

As Afghan forces grow and take over, Western nations are plotting their withdrawal.

Australian Prime Minister Julia Gillard said Tuesday she would bring home her 1,550 troops a year earlier than planned, with most soldiers withdrawn in 2013 after significant security gains over the past 18 months.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 part of the transition plan agreed by NATO and its partners.

"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," he said.

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

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

Socialist candidate Francois Hollande, favourite to defeat Sarkozy in the two-round election taking place on Sunday and May 6, wants to bring troops home by the end of this year.

With the war increasingly unpopular in the United States too, President Barack Obama, facing his own tough election in November, plans to withdraw 23,000 of the 90,000 US troops in Afghanistan this year.

The US military, meanwhile, was hit by a potentially damaging revelation on Wednesday as the Los Angeles Times published two-year-old photos of US soldiers posing with the remains of suspected suicide bombers.

Panetta vowed that those found responsible would be punished but he voiced "regret" that the LA Times published them against his wishes, saying they could prompt a backlash against US troops.

Rasmussen condemned the photos too but insisted it was an "isolated event."