As many as a third of votes cast for Afghan incumbent Hamid Karzai in last month's presidential election are suspect and must be checked for fraud, the head of a European Union election observer mission said.

The announcement by the largest foreign observer team in Afghanistan suggesting fraud on a massive scale came hours before authorities were due to issue a preliminary final tally expected to show Karzai with enough votes to win in a single round.

The fraud accusations have come at a particularly difficult time for US President Barack Obama, who has already ordered thousands of additional troops to Afghanistan and is expected to make a decision in coming weeks about whether to send more.

The war is already becoming increasingly unpopular at home, and Obama may find it more difficult to persuade US citizenns to send soldiers to die to defend a government whose legitimacy could be called into question.

The preliminary final result can still be overturned by a separate body, the UN-backed Electoral Complaints Commission, which has already ordered a recount of 10 percent of polling stations after finding "clear and convincing evidence of fraud."

Phillippe Morillon, head of the EU observer team, told reporters his team believed 1.5 million votes were suspicious, including 1.1 million cast for Karzai and 300,000 cast for his main rival, former foreign minister Abdullah Abdullah.

In near-complete figures issued last week, Karzai had slightly more than 3 million votes, or 54.3 percent of the 5.54 million valid votes counted.

Were all the votes described by Morillon as "suspicious" to be removed from that preliminary tally, Karzai would fall just short of the 50 percent needed to win in a single round, and would instead face a run-off against Abdullah.

Morillon said fraud had been carried out by "unscrupulous, overzealous supporters ... from every camp," and that fraudulent ballots needed to be removed from the count before any result could be considered final.

"Any claim for any count or of victory will be premature and not credible," he said.

Four weeks after the election, Afghanistan remains mired in political limbo, with results trickling out as fraud accusations mounted, undermining faith in the vote both in Afghanistan and among Western nations with troops fighting there.

The UN-backed ECC must sign off on any final result, and its fraud probe could potentially force a second round if it invalidates enough ballots to put Karzai below the 50 percent threshold needed to win in a single round and avoid a run-off.

The recount process has only just begun and could take weeks or even months.

A second round, if needed, would have to be held within two weeks of the final result being declared, although there has been some concern that this could be difficult if it is delayed into winter when travel is difficult in Afghanistan.