A suicide bomber killed 17 people and wounded another 21 in assassinating a police commander at a public bath in a southern Afghan town on the border with Pakistan, the local government said.
A nine-year Taliban insurgency is concentrated in southern Afghanistan, but suicide attacks in public places dominated by civilians are relatively rare.
"At around 12:00 pm (0730 GMT) suicide bomber blew up explosives strapped to his chest at a public bath in Spin Boldak," border police official General Abdul Raziq told AFP.
The local government confirmed from provincial capital Kandahar that at least 17 people had died in the attack and that 23 people were wounded.
The administration released a statement saying that the bomber had been targeting a police commander named Ramazan, who died in the attack.
Raziq identified him as head of the local rapid reaction police border unit.
There was no immediate claim of responsibility for the blast, but such attacks are trademarks of the Taliban, who are fighting to bring down the Western-backed Afghan government and expel 140,000 US-led foreign troops.
Spin Boldak is a short distance from the border with Pakistan, where US officials say the Taliban and Al-Qaeda-linked insurgents use rear bases to plot attacks in Afghanistan and the West.
US Defence Secretary Robert Gates this week ordered an extra 1,400 Marines to southern Afghanistan in what the Pentagon said would put pressure on the Taliban in order to pre-empt a traditional insurgent offensive in the spring.
Defence officials expressed hope that the extra troops would bolster areas recently cleared between Kandahar city and Helmand province.
There are about 97,000 American troops in Afghanistan and 45,000 from other countries. Officials said the new Marines would not push the total number of US forces above the limit of 100,000 authorised by President Barack Obama.
A year after ordering 30,000 more troops to Afghanistan has part of a last-ditch "surge" strategy designed to defeat Al-Qaeda and reverse the Taliban, Obama conceded last month that gains were fragile and reversible.
The White House said the strategy was "on track" but 711 foreign troops died in the Afghan during 2010, an average of two a day and exceeding by far the previous most deadly annual toll of 521 during 2009.
US commanders are under pressure to show clear progress in Afghanistan in 2011 so that at least a limited draw down of American troops can begin from July, or else face fresh public doubts about the course of the war.
US officials see the American campaign in the south as make-or-break for the war effort, pinning their hopes on undermining the Taliban in its heartland and limiting the number of attacks such as the bombing on Friday.
The United Nations said 2,412 Afghan civilians died in the first 10 months of 2010, an increase of 20 per cent on the corresponding period in 2009.
The Afghan interior ministry said 1,292 policemen died in the war last year.