Bringing Afghan presidential elections forward from August to April as demanded by President Hamid Karzai will not give other candidates time to contest the campaign, an Afghan presidential contender said.

Karzai's call on Saturday for the poll to be held much sooner than almost anyone sees as practically possible was a deft political manoeuvre, analysts say, to force opposition groups to let him stay in office after a May constitutional deadline.

The election commission earlier set Aug. 20 as the date for polls.

The United States "supports the underlying principles articulated by President Karzai" but still believed August would be a better time to hold elections in a secure environment, State Department spokesman Robert Wood said.

U.S. President Barack Obama has ordered 17,000 more troops to Afghanistan.

Other candidates would also be put at a disadvantage, said presidential contender Ashraf Ghani Ahmadzai.

"All candidates and influential figures have been trying to get ready for the campaign in the month of August, but a sudden change to the decision and holding the elections in the month of April will create certain problems," he told Afghan television late on Saturday.

Karzai's decree said the election should be held according to the constitution. That states the president's term ends on May 21 and new polls to elect his successor must be held between 30 and 60 days before that, giving April 21 as the last possible date.

Opposition leaders had said Karzai's position would be illegitimate if he remained in office beyond May 21.

The Independent Election Commission (IEC) said a spring election would be "impossible" because of harsh Afghan winter.

"Before deciding the election date, the Independent Election Commission took into account all aspects including funding, security and the wide participation in the polls, and also climate," IEC deputy chief Zekria Barakzai told Reuters.

He said the commission had not yet received an official copy of the decree. "We are waiting to receive the presidential decree and then we will evaluate it and make our decision," he said.

The Taliban rejected a call from Karzai for them to take part in the election.

"Not only will the Taliban not take part in the election, we also ask the Afghan nation not to participate in it as this is an infidel system and Afghanistan is now a country occupied by foreigners," Taliban spokesman Qari Mohammad Yousuf told the Pakistan-based Afghan Islamic Press.

"It is a fresh presidential decree and we are working on a plan," said Defence Ministry spokesman Zaher Murad.

Ahmadzai, a senior fellow in foreign policy at the Brookings Institution in Washington, also said only the Supreme Court had the right to interpret the constitution.

Hafiz Mansur, the head of one of the two main opposition groups, said his party believed in the constitution but it should not be exploited to "create chaos, disorder and hold the elections unfairly", he told Afghan television.

Ahmadzai called for a large meeting of Afghan political leaders to decide on how best to hold a free and fair election.

Agencies