It conflicts with a decision by the country's electoral commission to hold elections by 20 August.

Mr Karzai's presidential term is due to end on 21 May, creating the potential for a constitutional crisis if polls are held much later.

The president has no power to unilaterally choose election dates.

Electoral 'contradictions'
 

According to Article 61 of the country's constitution, elections should be held 30 to 60 days before 22 May, the end of Mr Karzai's five-year term.

International monitors had said it would be difficult to hold a fair election so soon, because of security concerns, bad weather and the logistical challenges of getting ballots.
 

The Independent Election Commission (IEC) said there was a contradiction between the constitution and electoral law which meant the president could stay in power until October - five years after he won the last election - or December, five years after he took his oath of office, Reuters news agency reported.
 

More US troops are expected to bolster security in the country

Hamid Karzai has been under considerable pressure over the delay and has been accused of using it to illegally extend his rule in breach of the constitution, says the BBC's Ian Pannell in Kabul.
 

Now he has put the onus for deciding when the vote should be held, and ultimately who runs the country in the event of a delay, back at the feet of the commission and his opponents, our correspondent says.
 

The US and other members of the international community supported the IEC's recommendation for an August poll, as the 17,000 foreign troops expected to bolster peacekeeping forces can be used to secure voting stations from the Taleban, reports say.
 

IEC chief Azizullah Ludin said that 20 August was chosen for the presidential polls after consultations with Afghan and international security forces.

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

Afghanistan continues to experience militant attacks and suicide bombings by the Taleban, who were ousted from power in the US-led invasion of 2001.