KABUL: Allegations of election fraud in Afghanistan soared Monday ahead of the fourth tranche of results from presidential polls in which incumbent Hamid Karzai is apparently cruising towards victory.

Officials were expected to announce results from around half of polling stations later Monday following the country's second presidential vote, which was overshadowed by an escalating Taliban insurgency and abysmal turnout.

Complaints of fraud are flooding in daily to Afghanistan's Electoral Complaints Commission (ECC), which is scrambling to investigate more than 2,500 allegations that fraud compromised the August 20 elections.

"We have received 691 complaints which are priority A," said EEC spokesman Ahmad Muslim Khuram, adding that 2,097 complaints had been received since election day, with the total since campaigning began in mid-June at 2,564.

The deluge of complaints threatens to compromise the legitimacy of the results and any victory for Karzai, whose seven-year rule has been marred by corruption, rising insecurity and cooling ties with his Western allies.

Priority A is the tag given to complaints that have the potential to affect the final outcome and which must be investigated before the ECC can certify the final results, which are not expected until mid-September at the earliest.

Out of 2.03 million valid votes released by the Independent Election Commission (IEC), which has come under criticism for favouring the incumbent, Karzai won 940,558 and former foreign minister Abdullah Abdullah 638,924.

The figures give Karzai 46.2 percent of votes announced and Abdullah 31.4 percent, according to the IEC website. Karzai needs 50 percent plus one vote to avoid a second round.

The results are being released in stages in a process that IEC spokesman Noor Mohammad Noor said would lead to the announcement of preliminary results between September 3 and September 7, and the final tally on September 17.

Noor said election officials expected to announce results from around 52 percent of polling stations later Monday, which include results from 35 percent of polling stations already made public.

"Until now we don't have the exact figure of participation in the election so we're just counting by polling station and we're just releasing to the media the percentage of the polling stations," he said.

The United States and its Western allies initially welcomed the elections as a success but fraud concerns and early results that point to turnout of 30-35 percent have raised questions about the legitimacy of the outcome.

Sweden's Foreign Minister Carl Bildt, whose country holds the EU presidency, stressed Western concerns about the election and hopes that it can provide a basis for political stability, as he headed for talks with Karzai on Sunday.

Abdullah, who has accused his opponent of rigging the vote, has said he would not accept a compromised outcome and would examine all legal avenues to counter what he called "state-engineered fraud".

Daily attacks and bombings highlight the challenges that the next president will face in combating an insurgency that has made 2009 the deadliest year for more than 100,000 foreign troops fighting against Taliban rebels.

A remote-controlled bomb on the side of a road ripped through a police vehicle in Karzai's home province of Kandahar, killing three policemen and wounding four on Sunday, Arghandab district governor Abdul Jabar said.

In the neighbouring Zabul, where the Taliban also have a strong presence, militants in provincial capital Qalat attacked a logistics convoy supplying NATO forces, killing one security guard, police said.

In northwest Faryab province, four Afghan police staged a dramatic breakout from a militant prison, killing six of their captors in a gunfight that left one officer dead, the interior ministry said.