* Presidential race set for Abdullah-Ghani run-off
* Election commission says second round will be on June 7
* Main concerns security, high cost and low turnout
* Delays due to fraud investigations (Recasts with final preliminary count, adds quotes)
By Jeremy Laurence and Mirwais Harooni
KABUL, April 26 (Reuters) - Afghanistan's presidential election is set to go to a second round between former foreign minister Abdullah Abdullah and ex-World Bank economist Ashraf Ghani after no candidate won an absolute majority, preliminary results showed on Saturday.
Abdullah finished top with 44.9 percent, followed by Ghani with 31.5 percent, the Independent Election Commission (IEC) said. Zalmay Rassoul, also a former foreign minister, was a distant third with 11.5 percent.
"This is a preliminary outcome and will now go to the Independent Election Complaints Commission and they will work on this. As soon as they share their findings with us we will also announce it," IEC chairman Ahmad Yousuf Nuristani told reporters.
The final result is due to be announced on May 14. In the meantime, authorities will investigate allegations of fraud involving up to half a million ballots.
It is unlikely, however, that Abdullah could be pushed to the 50 percent-plus-one required for victory if the suspect votes are cleared and included in the count.
"We have a tentative schedule of June 7 to start the second round," Nuristani said.
The April 5 election was widely seen as a success. Around 7 million of an eligible 12 million voters braved the threat of Taliban attacks to cast ballots in what will be the first democratic transition of power in their country's history.
President Hamid Karzai was constitutionally barred from standing for a third term. His successor will face a range of challenges, including leading the country to sovereignty after more than a decade of foreign military occupation that followed the U.S.-led invasion to rid Afghanistan of the Taliban in 2001.
Foreign combat troops are set to withdraw on Dec. 31, leaving security to Afghanistan's military and police, trained and funded by the country's Western supporters though their readiness to stand alone has been questioned.
Amid concerns that voter turnout will not be as high in the run-off as it was on April 5, both Abdullah and Ghani have dismissed suggestions that they strike a deal to avert a second round. They have said the democratic process should be completed, a sentiment echoed by the country's Western allies.
The U.N. envoy to Afghanistan, Jan Kubis, applauded the election. "The Afghan electoral institutions should be commended for their work to make the process more transparent than ever before," he said in a statement.
Some observers said they could see the merit of a deal to swiftly move on with the political transition, though there were doubts that Abdullah and Ghani would want to work together.
"We're seeing intense discussions among the most powerful men in Kabul," said Graeme Smith, a Kabul-based analyst with the International Crisis Group (ICG).
"I would be astounded if Abdullah and Ghani set aside their profound differences and cut a deal, but this is a season of surprises in Afghanistan."
There are also concerns about security and cost. The first round was funded by Washington, at more than $100 million. Repeating the process in which some ballot boxes are carried by donkey to and from remote regions means it could be July before a new president is officially declared.
The United States has not publicly backed a candidate, as the top contenders have said they will sign a security agreement allowing some U.S. troops to remain after the December drawdown deadline.
Washington's relationship with Karzai has sharply deteriorated over the past year, exacerbated by his refusal to sign a bilateral security agreement permitting a small force to stay for counter-terrorism support and training.
As the incremental release of preliminary results made it clear that Abdullah would not secure an outright victory, the leading candidates began lobbying in anticipation of a run-off.
Third-placed candidate Rassoul, long a confidante of Karzai, and Islamist politician Abdul Rassoul Sayyaf, who finished fourth with 7.1 percent, are emerging as the kingmakers.
Access to Rassoul's support base is seen as crucial as he is believed to have the backing of the powerful Karzai clan. Both Ghani and Abdullah have promised an advisory role for the outgoing president.
"Sayyaf is a wild card," said the ICG's Smith. The conservative Islamic scholar fought the Soviet occupation and was once close to Osama bin Laden.
"He showed surprising strength in the campaign and his voting bloc is likely to remain more coherent in a second round than the coalition that stood behind Rassoul - which gives Sayyaf some bargaining power as he sits down with the Abdullah and Ghani camps," Smith said.
Abdullah, an ophthalmologist who also fought Soviet forces in the 1980s, has already reached out to Rassoul. Ghani's camp has refused to detail its behind-the-scenes negotiations.
Sources close to Rassoul told Reuters they had met Abdullah and envisaged the two sides working well together on foreign policy matters and the peace process with the Taliban.
For years, the Taliban's leadership has refused to negotiate directly with the Karzai's government. The Taliban says Karzai is an illegitimate leader installed by the United States.
Sayyaf presents himself as a voice of wisdom and a bridge between warring factions, and says peace can be reached with the Taliban if they renounce outside influence. (Additional reporting by Hamid Shalizi; Editing by Lynne O'Donnell)