Image copyright Getty Images Image caption The education sector has become especially vulnerable to corruption, the report says

The cost of corruption has risen sharply in Afghanistan but fewer people are paying bribes, a UN report says.

It said the amount rose in 2012 to $3.9bn, twice the country's domestic revenue and that 50% of Afghans were paying bribes compared to 58% in 2009.

Increasing numbers of people say they find it acceptable for civil servants to take small bribes, the report adds.

The government blames the international community's system of giving contracts to officials for spreading corruption.

However, it accepts that the problem is rife within its own ranks.

The BBC's Bilal Sarwary says that what is revealed in this report could just be the tip of the iceberg.

It is also unclear to what extent respondents felt able to speak openly about the bribery and corruption they have to contend with, our correspondent says.

'Correct strategy needed'

The report was compiled jointly by the UN office on Drugs and Crime (UNODC) and Afghanistan's anti-corruption unit, based on a survey of 6,700 people,

It says that while there has been ''some tangible progress'' in the fight against corruption, the total cost of corruption rose to $3.9bn in 2012, a 40% jump over the 2009 figure.

According to the report, the bribes Afghans paid last year amounted to double the country's domestic revenue or one-quarter of the $16bn promised by donors for Afghanistan at a conference in Japan last year.

But corruption appears to be increasingly tolerated by ordinary people and "embedded in social practices", the report added.

More than 68% surveyed considered it acceptable for a civil servant to top up a low salary by accepting small bribes - up from 42% in 2009, it said.

"Afghans know that corruption is eating at the fabric of their society," said UNODC regional representative Jean-Luc Lemahieu.

"The solution is not only to be found within the government but also within the wider community."

The report noted that while the cost of corruption had risen, the total number of people paying bribes had dropped from 58% in 2009 to 50% last year - but they were paying more often.

It said that the education sector had become especially vulnerable, with the number of Afghans bribing teachers jumping from 16% in 2009 to 51% in 2012.

Afghan President Hamid Karzai has repeatedly called for greater efforts to deal with the problem.

He has appealed to the US and other countries not to give construction, reconstruction and business contracts to Afghan government officials or their relatives, which he says is making the problem worse.

UN officials said it was not a question of recognising the problem but finding the right way to do something about it.

"Nobody doubts the seriousness of the issue, the art is to design the correct strategy to remedy the situation. The findings of the survey will allow us to do so," Mr Lemahieu said.