Afghan election front-runner Abdullah Abdullah received a major boost to his presidential campaign Sunday when Zalmai Rassoul, a close ally of the current leader, endorsed him after withdrawing from the race.

Rassoul was seen as the favoured candidate of outgoing President Hamid Karzai, and his support for Abdullah could be decisive in deal-making to choose Afghanistan's next leader as US-led troops withdraw from the country.

Rassoul's own bid to become president gained little momentum and he came third with just 11.5 percent in the first round of voting, but he remains an influential power-broker.

"I ask the people for the sake of national unity and political stability to vote for Doctor Abdullah, so that we win the election," Rassoul, who resigned as foreign minister to run in the election, told a press conference in Kabul.

Rassoul said his support could be a "balance maker" in the run-off election due next month.

The second-round vote will pit Abdullah against former World Bank economist Ashraf Ghani in a head-to-head battle, after the two came first and second in the eight-man election on April 5.

Neither candidate secured the 50 percent needed for outright victory, with Abdullah gaining 44.9 percent and Ghani on 31.5 percent, according to preliminary results.

The official final first-round result will be announced on Wednesday after weeks of adjudication over fraud complaints.

The 2009 election, when Karzai retained power after defeating Abdullah, was marred by massive fraud in a chaotic process that shook the multinational effort to develop the country after the ousting of the Taliban regime in 2001.

The eventual winner will lead Afghanistan into a new era as US-led NATO combat troops end their 13-year war against the Taliban insurgency that erupted after Karzai took power.

The first-round election was hailed as a success by Afghan officials and foreign allies, with turnout far better than in 2009 and the Taliban failing to launch a major attack despite threats to disrupt the vote.

A run-off vote in June -- at the height of the traditional "fighting season" -- could be more difficult for Afghanistan's stretched security forces to guard.

Another election could be avoided by negotiations in the coming weeks, but both Abdullah and Ghani have dismissed talk of a power-sharing deal.

Karzai, who is constitutionally barred from a third term in office, stayed publicly neutral in the election. But diplomats in Kabul said Rassoul had received discreet backing from the president in the early stages of the campaign.