By Jessica Donati and Mirwais Harooni
KABUL, June 21 (Reuters) - The United Nations on Saturday urged Afghan presidential candidate Abdullah Abdullah to return to the electoral process after he dropped out earlier this week, accusing the organisers and the president of fraud.
Abdullah withdrew by declaring his camp would regard any outcome as illegitimate and recalling his observers from the vote count for last week's run-off election. He also invited the U.N. to intervene.
"We believe that the task ahead of us is to have the candidates re-engage fully in the electoral process," U.N. deputy chief Nicholas Haysom told reporters in Kabul.
"We would want to emphasize that there is no other way of electing a legitimate leader."
The run-off had pitted Adbullah against Ashraf Ghani, neither of whom gained the 50 percent of the vote needed to win outright in the first round of elections on April 5.
Abdullah's withdrawal has intensified longstanding concerns about a struggle for power along ethnic lines, casting doubt on Afghanistan's attempt to transfer power democratically for the first time in its history.
While the vote count is continuing, Abdullah's withdrawal has heightened tension across the country. At least one deadly gun battle erupted between rival supporters this week.
The election comes as most foreign troops are planning to leave Afghanistan by the end of the year. The fragile state of the society they will leave behind was underscored on Saturday by a suicide bombing aimed at a government official.
Ghani's team has said it is in favour of any process that increases the transparency of the electoral bodies but wants the election to remain under local control.
"We respect the role of the U.N. ... but any solution should be Afghan-led and shouldn't affect the work of the Independent Election Commission and Complaints Commission," said spokesman Abbas Noyan.
The commissions were heavily criticised in the first round for lacking transparency. Both candidates say they failed to properly adjudicate cases of fraud, allowing hundreds of thousands of fake votes to be included in the final tally.
However, Afghan officials and diplomats alike want candidates to give the electoral bodies a chance to prove they have reformed. A statement by President Hamid Karzai on Friday backing Abdullah's call for U.N. intervention was met with dismay by those advocating the institutions be respected.
"He has lost his marbles," said one Afghan official on condition of anonymity, who said he was seriously worried the electoral process would collapse.
Abdullah's supporters have organised protests in the capital this week and there are fears that demonstrations could turn violent and take on an ethnic dimension. Most of Abdullah's supporters are Tajiks, the second-largest ethnic group. Ghani's are mainly Pashtun, the largest group in Afghanistan.
So far, protests have been small, but they have taken on a nasty tone. Several hundred people gathered near the airport on Saturday, for example, where they chanted "we will defend our vote to the last drop of blood" and brandished banners with slogans like "Death to Karzai" and "Death to Ghani".
The tone of the debate on social media and in public has alarmed the U.N., among others. It has urged the candidates and the public to behave responsibly and avoid inciting ethnic divisions.
"Should any violence emanate from the demonstrations it could set back the process, make the task of trust-building more difficult," Haysom said, "It could lead to a spiral of instability." He called the use of social media to inflame divisions "disturbing".
The threat of violence was underscored on Saturday when a suicide bomber killed himself and a bystander in Kabul. Police said the bombing was an attempt to assassinate an advisor to the High Peace Council, which is seeking reconciliation with Taliban fighters.
The advisor, Mohammad Massom Stanikzai, escaped unharmed, but other several people were wounded, police officials said.
(Additional reporting by Praveen Menon; Writing by Jessica Donati; Editing by Simon Cameron-Moore, Larry King)