* Both candidates polling below 50 pct
* Preliminary results due Saturday, final outcome on May 14
* Run-off risky due to security concerns, costs, low turnout
* April vote hailed success, only minor attacks by Taliban (Adds details on fraud concerns, alliances, U.S. interest)
By Hamid Shalizi and Jeremy Laurence
KABUL, April 24 (Reuters) - The Afghanistan presidential vote is poised to go to a run-off, the election chief said on Thursday, after the latest tally of ballots showed neither Abdullah Abdullah nor Ashraf Ghani securing an outright majority.
Former foreign minister Abdullah and ex-World Bank official Ghani have taken an insurmountable lead, but both are polling well under 50 percent with just over 82 percent of the vote counted, setting the stage for a second round between the pair in late May.
"With the 17.5 percent of the votes that are going to be counted, it is doubtful that anyone will win in the first round," Independent Election Commission (IEC) chairman Ahmad Yousuf Nuristani told a news conference in Kabul.
The final preliminary results are due to be released on Saturday, but that figure will exclude votes being investigated for fraud, numbering up to half a million ballots. Final results are due on May 14.
The United States plans to withdraw most of its troops from Afghanistan by the end of 2014, so the longer the wait for a new leader to be installed, the bigger the risk of instability from a Taliban insurgency and from rivalry between ethnic or tribal factions.
Washington has not stated publicly a preference for a candidate, content the frontrunners have all stated they will sign a security agreement that will allow some U.S. troops to stay behind after a December deadline for foreign forces to leave.
U.S. relations with President Hamid Karzai had sharply deteriorated over the past year. One of the sticking points has been his refusal to sign the bilateral security agreement permitting the U.S. military to stay on to train Afghan forces.
After 12 years in power, Karzai is constitutionally bound to step down.
The two frontrunners, who both held high-profile ministerial positions in Karzai's first administration, have dismissed talk of a deal to avoid a second round, saying such a move would be unlawful.
A run-off is seen as a risky proposition in Afghanistan, given the security concerns, the prospect of a low turnout and the cost - the bill for the first round was put at more than $100 million.
Afghanistan's allies hailed the first round on April 5 a success due to the high number of participants and because Taliban militants had only staged a handful of minor attacks on polling day.
The eight candidates have all since cried foul, but the front-runners have said they would respect the final outcome. Abdullah quit the last presidential poll in 2009 election, citing massive fraud, cancelling a scheduled run-off against Karzai.
As it has become clear there would be no outright winner, the candidates have started horse-trading in a bid to secure votes going into the likely second round.
Abdullah, a trained ophthalmologist turned anti-Soviet resistance fighter, has reached out to Zalmay Rassoul, also a one-time foreign minister.
Access to Rassoul's support base is seen as crucial as he known to have the backing of the Karzai family. The outgoing president will retain his influence at the top levels of power. Ghani and Abdullah say they will reserve him a place on their team as an advisor.
Rassoul is from the majority Pashtun community while Abdullah's base of support is in the ethnic Tajik community, although he is half-Pashtun.
Abdullah was an adviser to the late guerrilla leader Ahmad Shah Masood, and then foreign minister of the United Front - better known internationally as the Northern Alliance - from 1998. After Masood's assassination in 2001 he became a dominant figure in the alliance that helped U.S. forces topple the Taliban.
Ghani, a Pashtun, is one of Afghanistan's best-known intellectuals, and spent almost a quarter of century abroad working at the World Bank and United Nations, as well as academic institutions.
The election commission's result sheet on Thursday showed Abdullah in the lead with 43.8 percent, followed by Ghani with 32.9 percent. Rassoul was a distant third with 11.1 percent. (Reporting by Hamid Shalizi and Jeremy Laurence; Editing by Robert Birsel and Raissa Kasolowsky)