Afghan election front-runner Abdullah Abdullah received a major boost to his presidential campaign on Sunday with the endorsement of third-placed Zalmai Rassoul, a close ally of the current leader, Hamid Karzai.

Afghan election front-runner Abdullah Abdullah received a major boost to his presidential campaign on Sunday with the endorsement of third-placed Zalmai Rassoul, a close ally of the current leader, Hamid Karzai.

Rassoul was seen as the favored candidate of outgoing Karzai, and his support for Abdullah could be influential 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 took just 11.5 percent in the first round of voting.

"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 for president, told a press conference.

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

The second-round vote will pit Abdullah against former World Bank economist Ashraf Ghani, 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.

Another costly, and potentially violent, election could be avoided by negotiations in the coming weeks, and Rassoul's support for Abdullah will increase pressure on Ghani to concede the race.

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