By Abdul Qadir Sediqi and Orooj Hakimi
KABUL (Reuters) - Afghanistan on Tuesday declared incumbent Ashraf Ghani winner of a disputed presidential election, but his main rival rejected the result and vowed to form his own government, threatening new turmoil as the United State strives to seal a U.S. troop withdrawal deal with Taliban militants.
Polls were held on Sept. 28 to select a president for the fourth time since U.S.-led forces overthrew the Taliban government in 2001. But the process was marred by allegations of rigging, technical problems with biometric devices used for voting, attacks and other irregularities..
Ghani won 50.64% of the vote, the Independent Election Commission (IEC) said on Tuesday. Abdullah Abdullah, Ghani's former deputy and main rival, was named runner-up with 39.52%.
Abdullah Abdullah nevertheless said he and his allies had won the election and would form the government.
"The result they (IEC) announced today was a result of election robbery, a coup against democracy, a betrayal of the will of the people, and we consider it illegal," he told a news conference following the announcement.
There was no immediate statement from the United States recognising Ghani as the election victor.
In Washington, a senior U.S. diplomat said that U.S. Special Representative Zalmay Khalilzad, who has led talks with the Taliban on a U.S. troop withdrawal agreement, had "fortuitously" arrived in Kabul and was speaking with Afghan political leaders.
Molly Phee, who is Khalilzad's deputy, told an event at the U.S. Institute of Peace think tank that the Ghani-Abdullah dispute "likely ... could add to the many challenges that Afghanistan faces, including the challenges associated with the peace process."
The IEC announced preliminary results in December in which Ghani, a former World Bank official, won re-election by a slim margin. Abdullah Abdullah dismissed the result as fraudulent and called for a full review. Ghani rejected the allegations.
The result echoes 2014, when both Ghani and Abdullah Abdullah alleged massive fraud by the other side, forcing the United States to broker an awkward power-sharing arrangement that made Ghani president and Abdullah Abdullah his chief executive.
The relationship has since been marred by distrust and a jostling for power in Kabul.
A potential political crisis looms as the United States and the Taliban near an agreement in Doha, which officials on both sides say could be announced by the end of February if an initial seven-day reduction in violence (RIV) is successfully observed. The RIV also would cover Afghan forces.
Afghanistan's acting interior minister said the RIV would be enforced within five days.
A U.S. troop reduction agreement would pave the way for talks between the political leadership and the Taliban on a settlement to decades of war. The militants have so far refused to talk to the government, which they have dismissed as a U.S. puppet.
The Taliban also rejected the result announced on Tuesday and termed Ghani's re-election to be against the peace process.
Ghani is seeking to appoint a team to negotiate with the Taliban. That effort has been mired in political wrangling and the dispute with Abdullah could add further complications.
In an apparent message to all Afghan factions, Phee said, "We believe we have established the conditions that can transform the trajectory of the conflict. It is high time for the parties to begin moving off the battlefield and into a political process."
Two Western diplomats in Kabul told Reuters the election result was vital.
"It was high time we got the results," one said on condition of anonymity. "All Western powers were deeply invested in the democratic process."
(Additional reporting by Rupam Jain and Jonathan Landay in Washington; Writing by Gibran Peshimam; Editing by Nick Macfie, Nick Tattersall and Bill Berkrot)