The Independent Election Commission (IEC) had declared in January that elections would be held on August 20.
Zekria Barakzai, the deputy chief electoral officer, said on Sunday: "The IEC considered all aspects of a free and fair election ... when it said [August 20] was the soonest date possible."
The IEC will review the decree issued by Karzai, which it had yet to receive officially, and then announce its reaction, Barakzai said.
James Bays, Al Jazeera’s correspondent in the Afghan capital of Kabul, said diplomats had told him that Nato member countries are concerned about Karzai's decree.
"[Nato members] believe that for the elections to be held in April would be a disaster and that it would be impossible to hold free and fair elections in that time frame," he said.
"This is in terms of trying to organise them in this war-ravaged country and also trying to organise security, given the volatile nature of the situation in the south and east of the country."
Supporters of Karzai say that the Afghan president has done his job in examining the constitution and applying an election within its terms, Bays said.
"[Karzai's supporters] say he looked at this August date and the constitution, which says quite clearly that his terms ends in May and elections have to be held before that, and told the IEC that the date they have chosen is unconstitutional."
Karzai's opponents have said that the decree is part of a campaign to manipulate the election in the incumbent’s favour.
"Opponents of Karzai say they may not be ready [to contest an election] by April and that Karzai ... will have an unfair advantage, with all the security apparatus at his disposal," Bays said.
The main opposition alliance, which comprises about 20 political groups, said Karzai may have issued his decree in an attempt to force a state of emergency that would see his term extended until the next election.
But Sayed Aqa Fazil Sancharaki, a National Front spokesman, said the opposition will take part in an election if it were to be held in April.
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.
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.