KABUL (Reuters) - Presidential polls in Afghanistan cannot be held next month as demanded by President Hamid Karzai and August 20 must remain the polling day as originally scheduled, the election commission said on Wednesday.

Karzai issued a decree at the weekend calling for the poll to be moved forward to April to comply with the constitution, a move that stirred up political turmoil at a time when the threat from the Taliban insurgency is steadily increasing.

Due to the weather, security, budgetary and technical problems that we have, the elections cannot be held on the basis of the presidential decree, said Azizullah Ludin, head of the government-appointed election commission.

A period of political horse-trading will now likely ensue. Opposition leaders will demand Karzai step down after May 21, as demanded by the constitution, or at least try to extract guarantees from the president not to use his office to campaign.

The United States, Afghan opposition groups and rival presidential contenders all back the original August date.

The constitution says the president's term ends on May 21 and polls must be held at least a month before that.

Rival leaders had agreed with Karzai 11 months ago that it was impractical to hold an election in the spring because that would mean organizing it during the harsh Afghan winter.

But when the polls were set for August 20, those same opposition leaders began to raise doubts about Karzai's legitimacy to remain in office after May 21.

Given the huge security problems in Afghanistan, few think polls can be organized in little over a month, so Karzai's move effectively called the opposition's bluff.

His decree is seen as a way of forcing opponents to recognize his legitimacy after May 21 and allow him to retain the advantages of office while he goes on the election campaign.

Ludin said at a news conference that, given the other factors and the commission's lack of preparedness for setting up polling stations on time, the elections would have to be held on August 20.

The United Nations special envoy to Afghanistan swiftly hailed the commission's decision, saying the original polling date was important for the political stability of the country and the legitimacy of its constitution.

The date would also provide the time needed by the commission and part of the international community involved in helping the polls with funding and security for it, a statement quoted envoy Kai Eide as saying.

NATO, which leads a 56,000-strong force in Afghanistan, welcomed the election commission's decision, saying the August date would give time to deploy 17,000 extra U.S. troops, plus other temporary reinforcements due just for the election.

This date will provide, from a purely technical point of view, time for the alliance to prepare properly to bring in the forces necessary and the capabilities necessary to provide the maximum possible support for the electoral process, said NATO spokesman James Appathurai.

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The squabbling among Afghanistan's political class only undermines its fledgling democracy as it battles the hardline Islamist Taliban militia.

Clashes this year have already risen past the record levels in the same period last year, the worst year of violence since U.S.-led and Afghan forces toppled the Taliban for sheltering the al Qaeda leaders behind the September 11 attacks.

A car bomb blast went off at the entrance of the main U.S. base in Afghanistan Wednesday, followed by a suicide bomber blowing himself up meters away, the U.S. military said.

Three U.S. contractors suffered minor wounds, it said.

Separately, three soldiers from the NATO-led force were killed by an explosion in southern Afghanistan Tuesday, the alliance said.

Military officials in Ottawa identified the three as Canadian soldiers.

There are already nearly 70,000 foreign troops under the command of NATO and the U.S. military in Afghanistan, where the al Qaeda-backed Taliban have regrouped since 2005.

The Taliban have vowed to disrupt the elections, the second one for choosing a president directly by poll in Afghan history.

Karzai, who has been leading Afghanistan since the Taliban's ouster in 2001, won a five-year term in office in 2004.

Once a darling of the West, his reputation has plummeted over poor governance.

(Additional reporting by Jonathan Burch in Bagram and David Brunnstrom in Brussels; Writing by Sayed Salahuddin; Editing by Paul Tait and Valerie Lee)

Afghanistan Elections