Under the country's constitution, the vote should have been held in May, but the deteriorating security situation has prompted a postponement.

The delay has come as little surprise to many observers. Large parts of the south and east are considered too unsafe for a free and fair vote.

Thousands of extra US troops are due to be sent to help improve security.

US President Barack Obama is considering almost doubling US forces in Afghanistan from 36,000 to more than 60,000 to ensure the polls are secure.

'Problems'
 

Independent Election Commission chief Azizullah Ludin said that 20 August was chosen for the presidential polls after consulting with Afghan and international security forces.
 

Security, budgetary and technical problems - you can see how many problems we have here   Azizullah Ludin, Election Commission head

"They told us there will be new security forces here... and they will guarantee security," Mr Ludin told a news conference in Kabul.

"Without security there can be no election," he said. "We have had security problems, budgetary problems and technical problems."

Mr Ludin said that without a delay it would not be possible to declare candidates, print ballots and distribute them nationwide - especially in mountainous areas during winter - in time for an April election.

He said that the commission was still short of the $223m required to hold the presidential and provincial council votes.

The BBC's Ian Pannell in Kabul says even the process of registering people to vote has had to be put on hold in some districts.

The hope is that the injection of thousands of extra US troops will create a safer environment for elections to take place, our correspondent says.

It will be only the second presidential election Afghanistan has held.
 

'No guarantee'

"We have always said it is important that elections should be held this year and we are pleased that the decision has now been taken," Nato spokesman James Appathurai said in response to the delay.

"The date chosen will give us sufficient time to properly prepare to support the Afghan government in ensuring enough security for the elections to go forward."

But some leading political figures in Kabul are sceptical that this can be done in such a short space of time.

A member of parliament in Afghanistan's National Assembly, Shukria Barakzai, told the BBC that unless security in the country improved it was possible the election would be further postponed.

"There is still no guarantee it will be a fair and fine election in August. In some of the districts which are still out of control of the government of Afghanistan, how can we say that there will elections for them?"

There are a number of potential challengers to President Hamid Karzai, whose popularity has steadily fallen.