Story highlights More than 40 killed in Paktika market attack

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

A car bomb exploded in a busy market in eastern Afghanistan Tuesday, killing more than 40 people, officials said, in what is said to be the single worst attack in the country this year.

Afghan Deputy Interior Minister Mohammad Ayoub Salang said 42 people were killed and more than 30 others were wounded in the attack in the Urgon district of Paktika province. The United Nations Assistance Mission in Afghanistan had different figures, stating that at least 43 civilians, including eight children, were killed, and 67 were wounded.

The car was driven by a suicide bomber who detonated explosives after police ordered him to stop his car, said Nissar Ahmad Abdulrahimzai, deputy police chief of Paktika province. Abdulrahimzai said he feared the death toll would rise and there is at least one government official reporting more deaths.

The United Nations, which calls the assault a suicide attack, says it's the single deadliest strike this year in the country.

"Today's appalling attack during Ramadan -- an occasion that should observed in a spirit of peace and compassion -- should be condemned in the strongest possible terms and the perpetrators must be held accountable," UNAMA head Jan Kubis said.

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The Taliban denied involvement.

Another attack in Kabul

Meanwhile, in the capital, Kabul, the Taliban claimed responsibility for an attack early Tuesday on a vehicle used by presidential palace employees.

Taliban spokesman Zabihullah Mujahid said in a statement to media that three people were killed and six others injured in the explosion.

Hashmatullah Stanekzai, a spokesman for the Kabul police chief, earlier told CNN that at least two people were killed and five injured. He said all the victims were working in the presidential palace media department.

Security remains a major concern in Afghanistan, from which the United States is due to withdraw most of its troops by year's end.

The latest violence comes amid disputes over the country's recent presidential election.

During a visit to Kabul on Saturday, U.S. Secretary of State John Kerry announced that an audit of votes would be carried out to decide the outcome of the election.

Both candidates, Abdullah Abdullah and Ashraf Ghani, have alleged large-scale vote fraud and manipulation during the runoff last month.