KABUL - EU election observers said Wednesday that around 1.5 million votes cast in Afghanistan's troubled elections last month could be fraudulent.

"We have calculated 1.5 million suspicious votes," said Dimitra Ioannou, the deputy head of the EU Election Observation Mission to Afghanistan.

She told reporters 1.1 million votes cast for the incumbent Hamid Karzai were suspicious and 300,000 for his main rival Abdullah Abdullah. The remainder of the suspicious votes were cast for other candidates.

The August 20 election has been overshadowed by allegations of widespread fraud that are threatening to undermine the final result.

Preliminary results are due to be released Wednesday, nearly a month after the election, although the final declaration of a winner is not likely for some weeks yet, officials say.

The EU observers did not give an estimated figure for turnout, expected to be low owing to intimidation of voters by the Taliban, whose insurgency against the Afghan government and Western troops is at its deadliest in eight years.

Karzai is leading an incomplete count with 54.3 percent, ahead of his main rival Abdullah on 28.1 percent.

Analysts have said that based on the count so far, turnout could be around 30-35 percent, or not much higher than six million out of a total registered electorate of 17 million.

Such figures -- including the EU estimate of suspicious votes -- could question up to 20 percent of the ballots, which could in turn impact Karzai's current lead over his main rival.

Electoral officials have already warned that hundreds of thousands of votes could be quarantined for investigations, which could take two to three weeks, delaying the announcement of the victor.

The winner must have 50 percent of the final tally, plus one vote in order to avoid a run-off between the two leading candidates.

The Independent Electoral Commission (IEC) plans to announce preliminary results from all polling stations on Wednesday, an official said.

"Today we will announce the preliminary results of the presidential election, based on 100 percent of the polling centres," IEC spokesman Abdul Rahim Nawakhtyar told AFP.

"We will also announce the turnout and other figures. All details will be included," he said.

Asked about investigations into allegations of vote-rigging and whether suspicious votes will be included in the tally, he said: "All I can say is that the preliminary results will be based on the qualified polling stations."