NEW : More than 40 killed in Paktika market bombings . December : The U.N. says it 's the single worst attack this year in Afghanistan . The bomb was planted by a suicide bomber , a government official says . Afghanistan continues to face major security threats .
NEW : More than 40 killed in Paktika market attack . NEW : The U.N. says it 's the single worst attack this year in Afghanistan . The device was detonated by a suicide bomber , a police official says . Afghanistan continues to face major security challenges .