More than 40 people were killed and dozens injured after a suicide bomb blast at a volleyball game in Afghanistan on Sunday. (Reuters)

More than 40 people were killed and dozens injured after a suicide bomb blast at a volleyball game in Afghanistan on Sunday. (Reuters)

A suicide bomber set off his explosives vest Sunday in a large crowd at a volleyball tournament in eastern Afghanistan, killing more than 40 people and injuring scores in one of the nation’s deadliest assaults this year, Afghan officials said.

No group asserted responsibility for the bombing, although suspicion immediately fell on the Taliban. In recent weeks, the insurgents have escalated their attacks in Kabul and elsewhere, targeting the capital’s heavily fortified police headquarters, compounds of foreigners, and an outspoken women’s rights activist, who barely escaped assassination.

The assault came a day after reports surfaced that the Obama administration had expanded the U.S. military’s role next year in combating the Taliban and other militants — and hours after Afghanistan’s parliament green-lighted a bilateral security pact allowing roughly 12,000 U.S. and international troops to remain past the end of this year.

The lone suicide bomber, Afghan officials and commanders said, waded through the densely packed crowd, filled with young men and children watching the last match of a tournament in the Yahyakhel district of Paktika province, which edges up to the border with Pakistan. It was shortly after 5 p.m., and fans were eagerly anticipating the outcome in a sport popular nationwide.

“It was the final volleyball match, and it was about to end when suddenly a man showed up in the crowd and in a loud voice said, ‘How are you guys? Allahu Akbar!’ ” said Gen. Sharif Yaftali, a senior Afghan army commander in the area. “Then he blew himself up.”

As many as 49 people were killed, he said, including two children. The blast wounded 63, and many of the injured were transferred to Kabul for treatment, he said. Other officials placed the death toll at 45 but said the number of casualties is expected to rise. The dead included Afghan police officers on patrol at the event.

Sunday’s attack unfolded in one of the most violent regions of the country. Taliban militants are active in Paktika, battling Afghan security forces for control of territory. The province is nestled along a porous border with Pakistan. Militants from Pakistan, including the Haqqani network and the Pakistani branch of the Taliban, have used the province as a base.

Earlier this year, in another district of the province, a suicide bomber blew up a car packed with explosives near a bustling market, killing scores.

The Taliban and other militants oppose Afghanistan’s new, Western-educated president, Ashraf Ghani, who took office nearly two months ago. Unlike his predecessor, Hamid Karzai, Ghani is forging a close relationship with the United States and other Western powers. One of his first acts was to sign the bilateral security agreement allowing American and other foreign troops to remain in the country past the end of the year, which the Taliban has condemned.

More recently, Ghani reached an understanding with the Obama administration to allow U.S. troops to engage the Taliban and other militants if they pose a threat to American soldiers or the Afghan government next year — and to provide air support to Afghan security forces when needed.

The previous deal called for U.S. forces to remain only in a training and advisory capacity next year, though they would continue to conduct counterterrorism operations against al-Qaeda. But with the attacks rising and casualties mounting among Afghan security forces, Ghani and the U.S. military came to the conclusion that a more active American engagement was needed next year if the Taliban is to be defeated.

Mohammad Sharif contributed to this report.