Ahead of the exit of foreign combat troops, Afghanistan faces pressure to tackle pervasive corruption as it seeks billions in new aid at an international conference in Tokyo on Sunday.

Tens of billions of dollars have poured into Afghanistan since the US-led invasion that toppled the Taliban in late 2001, but graft is rife from local police to high officials, and patience among donor countries is wearing thin.

Afghanistan wants to see around $4 billion a year in civilian assistance pledged in Tokyo for its aid-dependent economy, amid fears that donations could dry up when NATO pulls out in 2014.

But a principle of "mutual accountability" will be stressed at the 70-nation meeting, making continued payment of aid conditional on Kabul making progress, particularly on transparency.

After more than 30 years of war, the Afghan economy is weak and the country cannot survive without foreign aid. According to the World Bank, spending on defence and development by donors accounted for more than 95 percent of GDP in 2010-11.

Without a functioning economy, Kabul covers only $2 billion of the $6 billion it spends each year not counting security costs, said a Western diplomat, with donor countries making up the difference.

President Hamid Karzai, who will be in Tokyo along with officials including US Secretary of State Hillary Clinton and United Nations chief Ban Ki-moon, has called for $4 billion a year in civilian aid for Afghanistan.

That would add to the $4.1 billion promised annually at a Chicago conference in May for security costs.

The Western diplomat said the Afghans were terrified that when NATO pulls out, the money will disappear with them.

Sources expect a deal worth up to $3.9 billion a year to be agreed in Tokyo, but after more than 10 years of sacrificing soldiers and tax dollars to the Afghan cause, leading donors are proving hard to persuade.

"We are not blind. We feel a considerable fatigue among the taxpayers," said another diplomat.

European Union ambassador Vygaudas Usackas said the bloc was "committed to continue to prioritise the support to Afghanistan in the coming decade, enhancing our overall support post-2014".

But the money will come with strings attached.

A European diplomat said work was needed in five key areas: better management of public finances; improved tax collection; guarantees on rights, particularly for women; legal reforms; and "credible" elections in 2014.

"Without tangible progress in these five areas, it will be difficult for donors to maintain their support to Afghanistan," the diplomat said, speaking on condition of anonymity.

Japanese Foreign Minister Koichiro Gemba, who will jointly chair the Tokyo conference, said he was hoping it would result in pledges worth at least $3.0 billion a year.

But in an interview with the Asahi Shimbun newspaper published Friday, he also warned of conditions for Karzai's government.

"(Kabul) must improve its governance capacity, including eradicating corruption," he said, adding a mechanism to review progress in these areas every two years had to be developed.

But there is resistance from the Afghans, who regard such conditions as attacks on their sovereignty.

"Many times, the international community wanted to tell us what to do or not and how to do it. But the Afghan government can only be fully responsible if it's able to make its own decisions," said a senior Afghan government official.

Aid organisations are also worried about what will happen to aid after 2014.

Since 2001, life expectancy has risen from 47 to 62 years for men and from 50 to 64 for women, according to Oxfam, which warned the good work of the last decade could be undone.

"Development gains made in Afghanistan over the last decade are in danger of being thrown away if levels of aid fall away in conjunction with the withdrawal of international troops in 2014," the British aid group said.

Oxfam said the United States, Afghanistan's biggest single donor, has already cut development aid by nearly half in 2011, from $4.1bn to $2.5bn.