KABUL (AFP) - Afghanistan's voting authority and the United States reiterated Sunday that presidential elections should be held in August, despite President Hamid Karzai's order for them to be held in April. With a showdown looming about a date for Afghanistan's second-ever presidential polls, candidates also said it would be impossible to prepare and campaign for an election in just weeks. The Independent Election Commission stood by its reasons for an August 20 election, but said it would review Karzai's order in a decree issued on Saturday. "The IEC considered all aspects of a free and fair election in view of a large participation of voters and considering security, climate, funding and operations when it said (August 20) was the soonest date possible," Deputy Chief Electoral Officer Zekria Barakzai told AFP. The commission announced in January that it needed until August to prepare for the vote, with security problems dogging areas affected by a Taliban-led insurgency and winter conditions delaying preparations in other places. The date is later than that stipulated by Afghanistan's constitution but this was overridden by the need to conduct credible and free elections, it said at the time. The date was supported by Afghanistan's international backers with the United Nations saying it was a "pragmatic necessity." But Karzai's decree Saturday told the IEC to be ready to hold an election according to the constitutional timeline, which says the vote must be held 30 to 60 days before his five-year term expires on May 21. This would safeguard both the constitution and Afghanistan's achievements in the seven years since the war-torn nation adopted democracy following the fall of the Taliban regime, he said. The United States - the main supplier of troops and aid to post-Taliban Afghanistan - said it supported the "underlying principles" behind Karzai's call for an early election. However, August elections are "the best means to assure every Afghan citizen would be able to express his or her political preference in a secure environment," it said in a statement reissued in Kabul Sunday. US Secretary of State Hillary Clinton meanwhile discussed the date with Karzai by telephone late Saturday, the Afghan leader's office said Sunday, without giving details of the conversation. Karzai's order came after weeks of consultations about the vote and with the president apparently seeking to head off pressure to step down before the elections, which he has said he would contest. If he ends his term as set by the constitution and elections are held only in August, a mechanism will have to be found to fill a leadership vacuum in a fragile and unstable country. Nonetheless, an April election is seen as too soon for candidates to register and campaign, with serious challengers for the job - including former interior minister Ali Ahmad Jalali - yet to announce they intend to stand. "The candidates have even not yet submitted their papers to the election commission," said presidential candidate Anwar-Ul Haq Ahadi, who resigned last month as finance minister so that he could run. "The date he has set is not possible. I strongly oppose it," he said in a television interview. The NATO-led force, which is helping the government fight the insurgency, said last week that it requested from its members thousands of troops to help secure the poll but that the reinforcements had not yet been finalised.