KABUL — A figure equal to twice the Afghan government's domestic revenue — $3.9 billion — was gouged from the country by public sector corruption last year, a U.N. report said on Thursday.

This amount is also roughly equal to the annual aid pledged to Afghanistan until 2015 by the international community at last year's Tokyo Conference.

The report, by the U.N.'s Office on Drugs and Crime (UNODC), will lead to concerns about the mismanagement of those donor funds by Afghan officials in a country which has been consistently ranked as one of the world's most corrupt.

"While corruption is seen by Afghans as one of the most urgent challenges facing their country, it seems to be increasingly embedded in social practices, with patronage and bribery being an acceptable part of day-to-day life," the report said.

Almost 7,000 Afghans were surveyed last year and they revealed that corruption in the country had risen by 40 per cent since 2009. Half the population had to pay at least one bribe to a public official in 2012, the report said.

U.S.-backed Afghan President Hamid Karzai has repeatedly pledged to curb corruption in order to keep attracting aid from international donors as a planned transition from U.S.-led NATO forces to Afghan leadership takes place by the end of 2014.

But capital flight has continued, threatening Afghanistan's fragile economy and stability. The Afghan central bank estimates that the total amount of cash leaving Afghanistan each year could be as much as $8 billion.

The report found corruption was becoming a way of life in Afghanistan, with 68 percent of those surveyed saying it was acceptable for a civil servant to top up their salary by taking bribes.

However, it found that the percentage of people who paid a bribe had dropped from 59 in 2009 to 50 last year.

In September, Karzai sacked five provincial governors and made changes to almost a third of the country's 34 provinces in a shakeout of corrupt and inept officials aimed at soothing foreign donors' fears.

At last year's Tokyo Conference, delegates from 80 nations and international organizations pledged $16 billion in aid to the country over four years.

Afghanistan is plagued by corruption as a result of more than thirty years of war and nepotism. Last year it was ranked at the bottom of Transparency International's annual corruption index, sharing the spot with North Korea and Somalia.