Afghan President Hamid Karzai is trying to outmaneuver his opponents by calling for an early presidential election and he must be prevented from using his position to manipulate the poll, a rival said on Monday.

"I will not participate in sham elections in April," former finance minister and presidential contender Ashraf Ghani Ahmadzai said.

On Saturday, Karzai sprung a surprise by saying the poll should be brought forward to April from Aug. 20, setting him at odds with his own election commission and with his Western backers, who see a meaningful early poll as almost impossible.

The call was seen as a ploy to show he respected a May 21 constitutional deadline to leave office, but at the same time force his opponents, none of whom are remotely ready to stand in April, to ask the president to stay on.

The election commission is expected to respond to Karzai's decree next week and repeat its position that holding polls before August is impractical, if not impossible.

A period of political turmoil may follow, analysts say, in which Karzai may have to make concessions to stay in power.

Ahmadzai said Karzai was already using the machinery of state to give himself an unfair advantage in the election, with ministers actively supporting his campaign.

Presidential hopeful Hedayat Amin Arsala, who is also a senior adviser to Karzai, echoed the comments.

"Every effort should be made so that the incumbent encourages the government, members of the government to help ensure the elections are clean and transparent," he said. "If anyone tries to play with these things it will bring about instability."

The United States has said it believes August would be a better time to hold an election in a secure environment.

U.S. President Barack Obama has ordered 17,000 more troops to Afghanistan to try to secure the election in August against a powerful and growing Taliban insurgency. Bringing the poll forward would not give them time to arrive in the country.

Speaking to reporters in Washington, U.S. State Department spokesman Gordon Duguid declined to take a position on the timing except to say it "must be resolved in a way that leads to credible and secure elections accepted by the voters."

TALIBAN OPPOSE "INFIDEL SYSTEM"

The Taliban has denounced the democratic process as an "infidel system," and a former senior Taliban official said the movement would try to prevent the election taking place in swathes of the east and south of the country under its control.

Mullah Abdul Salam Zaeef, the former Taliban ambassador to Pakistan, said ordinary people were already tiring of Western-style democracy, which had failed to bring peace.

"The Taliban will try to create obstacles about the holding of elections," the former Guantanamo detainee, now living under guard in Kabul, told Reuters. "But the Afghan people are not very interested in taking part in elections anyway."

Ahmadzai was more optimistic, saying there were signs of significant voter registration in some areas controlled by Taliban, evidence perhaps of some debate within the movement about whether to oppose the poll.

The biggest obstacle to a fair election was a "corrupt and criminalised" police force, he said.

Almost no one believes an April poll is possible, especially because it would have to be organized during the harsh Afghan winter when many parts of the country are inaccessible.

NATO's 56,000-strong force is due to back up the Afghan army and police whose job it is to provide security for the polls.

"The bottom line is that we will be able to provide more support and more security for an August election than we would for an earlier one," NATO spokesman James Appathurai said.

Karzai is unpopular due to his failure to bring security and development, to tackle corruption and prevent civilian casualties in the U.S.-led war against the Taliban.

A senior U.S. official, speaking in Brussels, said the deployment of additional U.S. troops to help protect the election had been planned for the end of July.

"To have a successful election, one in which the Afghan people are fully able to participate and express their view, we need to have a more stable and secure environment," he said.

"The worst thing for Afghanistan would be to have an election that descends into violence and where the people don't have a chance to express their views."