A suicide bomber killed at least 16 people at a crowded market in northern Afghanistan on Tuesday, officials said, despite a tightening of security for presidential elections less than three weeks away.

There was no immediate claim of responsibility for the attack in Maimanah city, the capital of Faryab, a remote province that borders Turkmenistan and has a mixed population of Uzbek, Turkmen and Pashtun ethnic groups.

A week ago Taliban insurgent leaders vowed to target the presidential election, urging their fighters to attack polling staff, voters and security forces before the April 5 vote to choose a successor to Hamid Karzai.

“It was a suicide bombing in the middle of Maimanah city during the Tuesday bazaar,” provincial governor Mohammadullah Batash told the news agency.

“The blast happened on the main roundabout, which was very crowded. The bomber used a three-wheeler packed with explosives.”

Abdul Ali Haleem, the provincial health director, said that 16 people had died and 40 were treated for injuries, among them a pregnant woman and two children aged six and seven.

Northern Afghanistan is generally more peaceful than the south and east of the country, but Islamist insurgents, rival militias and criminal gangs are active in some districts.

Six Afghan employees of the aid group ACTED working on rural development projects were shot dead in Faryab in December by suspected Taliban gunmen.

The United Nations envoy to Kabul warned on Monday that election-related violence was on the rise in Afghanistan, where foreign combat troops are withdrawing from the country after 13 years of fighting a fierce Islamist insurgency.

“Security will have a major impact on these polls,” Jan Kubis said in an address to the Security Council in New York, adding he was “gravely disturbed” by the Taliban threat to unleash “a campaign of terror”.