Afghan officials said they had delayed releasing further results from the nation's disputed presidential poll, adding further confusion to a contest which appears headed for a second round.

Votes from about 17 percent of 27,000 polling stations have been counted so far, meaning results could swing wildly in the coming days. Preliminary final results are due on September 3, with the final tally about two weeks later.

Afghanistan has been in political limbo since the August 20 vote, with partial results released so far showing President Hamid Karzai leading his main rival, former foreign minister Abdullah Abdullah, by about 43 percent to 34 percent.

Afghanistan's Independent Election Commission (IEC) said only votes from provincial council elections, held concurrently with the presidential poll a week ago, would be counted on today.

Adding to an already chaotic picture, computer software failures meant counting was going slower than expected, said IEC deputy head Zekria Barakzai.

The election is a major test for Karzai after eight years in power and for US President Barack Obama, who has poured in thousands of extra troops as part of his new regional strategy to defeat the Taliban and stabilize Afghanistan.

The picture will not become any clearer for at least two days, with no counting planned for tomorrow, a Muslim holiday.

"We will come up with new figures and information on Saturday," Barakzai told reporters.

To avoid a potentially destabilising second round run-off in October, a candidate must win more than 50 percent of the vote.

The most likely outcome that can be gleaned from the partial election results released so far is that a run-off is likely, election observers say.

Security in Afghanistan has deteriorated rapidly this year, with attacks reaching their worst levels since the austere Islamist Taliban were ousted by U.S.-led Afghan forces in 2001.

Violence spiked further in the weeks leading up to the poll, which the Taliban vowed to disrupt, testing Obama's new strategy and softening support for the war in the United States, but the election went ahead with relatively few attacks.

However, at least 43 people were killed in a huge truck bomb blast on Tuesday in southern Kandahar city, the worst incident of its kind in more than a year. The Taliban denied responsibility for the blast.

With civilian and military casualties at record levels, 2009 has become the deadliest year of the war for foreign troops.