It remains to be seen now whether the electoral body would heed Karzai's request and bring forward the election date.
The US state department issued a statement saying it believes an August election "is the best means to assure that every Afghan citizen would be able to express his or her political preference in a secure environment".
Barack Obama, the US president, has ordered 17,000 more troops to deploy to Afghanistan, but the increased numbers would likely not be in the country if elections were to be held in April or May.
Karzai's office said that the presidential decree was made after discussions with political leaders and legal experts.
Some Afghan MPs say they would not recognise Karzai as president beyond May.
But other members of the parliament have criticised the president's move as unacceptable.
Dawood Sultanzoy, an Afghan MP and a presidential candidate, said that Karzai had read the constitution in such a way as to suit his personal ambitions.
"The president is using the constitution as a cafeteria - whenever he chooses, he wants to pick those articles out and side with them," he said.
"He very conveniently forgot article 59 of the constitution, which says that any individual who abuses the trust and the sovereignty of the people for his own interests is in violation of the law."
Opponents of the president have said the move is an effort to wrong-foot them.
Al Jazeera's James Bays, reporting from Kabul, said presidential candidates had been preparing for an August poll.
"Some of them haven't actually declared that they are candidates yet, they haven't got their campaigns in place [and] they say they won't be ready to fight an election by April," he said.
"What's more, will it be possible to even organise an election in a matter of weeks?"
Karzai's move also comes amid deterioration in relations between Washington and Kabul, ostensibly over how to tackle opposition fighters linked to the Taliban, al-Qaeda and tribal leaders.
Bays said that after seven years of Karzai's rule, some in the international community see him "as part of the problem rather than part of the solution".