* At least 15 dead weeks before crucial election
* Two children dead, United Nations says (Adds details, background)
By Mirwais Harooni
KABUL, March 18 (Reuters) - A suicide bomber killed 15 people in an attack on a busy marketplace in northern Afghanistan on Tuesday, officials said, despite attempts by the authorities to reassure Afghans over security during next month's presidential election.
The blast happened as Afghan security forces prepare to secure thousands of election sites across the country before the April 5 vote, designed to mark the first time one elected government hands power to another in the history of Afghanistan.
The Taliban have threatened to kill anyone who takes part in the elections, and eight people involved in political campaigning have been killed since electioneering started last month. A group of election officials has also been kidnapped.
At least 47 people were wounded when the suicide bomber driving a three-wheel rickshaw blew himself up in Maimana, the provincial capital of Faryab province, the Health Ministry said in a statement.
"It was a bazaar day and everybody was busy buying or selling when the bomber detonated his explosives," Faryab governor Mohammadullah Batash told Reuters.
Two children were among the dead, the United Nations said.
Nicholas Haysom, the top United Nations official in Afghanistan, said such bombings could be a war crime.
"Their use in a distinctly civilian location such as a market is atrocious and cannot be justified," he said in a statement.
The United Nations said such bombs - called improvised explosive devices - have killed 190 civilians in Afghanistan so far this year, a 14 percent increase from the same period last year.
President Hamid Karzai, who is due to stand down after serving two terms as president, condemned the attack.
Karzai has not officially backed any of the nine candidates remaining in the race, but his two brothers are campaigning with former Foreign Minister Zalmay Rassoul, cementing the official's reputation as a palace favourite.
The other two front-runners are Abdullah Abdullah, an eye surgeon and senior aide to a slain anti-Taliban militia leader, and Ashraf Ghani, an former World Bank official who has teamed up with another militia leader blamed for human rights abuses.
NATO troops are offering logistical support to Afghan security forces, who are responsible for securing polling stations and transport hubs on the day of the vote.
Thousands of observers will be deployed across the country in an attempt to prevent the repeat of the widespread fraud that marred the 2009 presidential polls when Karzai was reelected. More than a million votes were eventually discounted. (Reporting by Mirwais Harooni and Katharine Houreld in Kabul Bashir Ansari in Mazar-e-Sharif; Writing by Jessica Donati; Editing by Maria Golovnina and Angus MacSwan)