Survivors of a horrific suicide attack which killed 57 people at a volleyball game in Afghanistan told Monday how a bomb packed with ball bearings ripped through spectators enjoying the final moments of the match.

In the country's deadliest single attack since 2011, the bomber detonated his explosives as hundreds of young men and boys attended a tournament on Sunday featuring three local teams in the volatile eastern province of Paktika.

Separately, two NATO soldiers were killed in an attack in the east on Monday morning, the coalition said.

Paktika provincial spokesman Mukhlis Afghan said the death toll from Sunday's blast rose to 57 after 15 people died of their injuries overnight.

"The game was about to end when we heard a big bang," Salaam Khan, 19, told AFP at a military hospital in Kabul where he was flown for treatment to his injured chest and right leg.

"I was shouting for help. Just beside me was a dead army officer," he said. "There were local police and commanders watching the game. I saw some killed and wounded."

The attack underlines the challenges facing President Ashraf Ghani, who came to power in September, as US-led NATO troops wind down operations and Afghan security forces take over full responsibility for fighting the Taliban and other insurgents.

Afghanistan's intelligence agency named the Haqqani network, a hardline militant group aligned with the Taliban, as being behind for the blast.

The Haqqani network, which was designated a terrorist organisation by the US in 2012, has been blamed for large-scale attacks on government and NATO targets across Afghanistan as well as for many kidnappings and murders.

- 'Covered in blood' -

"I was watching the game, sitting on the ground with my brother, when the blast happened," said Mohammad Rasoul, 11, who was wounded in the chest and whose brother is in intensive care.

"People were covered in blood all around me. There were many friends of mine among them."

Many of the wounded are children and young men, and were wrapped in bloody bandages.

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Doctor Seyawash, head of health services at the hospital in Kabul, told reporters that about 12 victims were in a critical condition, explaining that the injuries were mostly sustained from ball bearings packed in the bomb.

The blast, in the Yahya Khail district of Paktika, came early on Sunday evening as crowds peaked at the volleyball, a popular sport in Afghanistan.

"I arrived after the bombing, it was an emergency situation. People were rushing the dead and wounded into cars," said Ghulam Mohammad, 60, whose injured grandson cradled a teddy bear in hospital.

- 'Inhumane, un-Islamic' -

"I went looking for my son and grandson. My son was fine but my grandson was wounded and we came to Kabul in a helicopter."

President Ghani visited some of the wounded at the 400-bed military hospital in the Afghan capital.

He condemned the attack as "inhumane and un-Islamic", saying "this kind of brutal killing of civilians cannot be justified".

Abdullah Abdullah, who signed a power-sharing deal with Ghani after disputed elections, visited elders in Yahya Khail district on Monday, but security concerns kept him away from the attack site.

Paktika borders Pakistan's lawless tribal areas, where many insurgent leaders seek refuge from NATO and Afghan forces.

Paktika was also hit by a massive suicide blast in July, when a bomber driving a truck packed with explosives killed at least 41 people at a market in Urgun district.

In April last year 46 people were killed in a Taliban attack on the law courts in the western province of Farah.

A suicide blast at a shrine in Kabul on the Shiite holy day of Ashura in December 2011 killed 80.

Sunday's attack occurred on the same day that the lower house of parliament approved agreements to allow about 12,500 NATO troops to stay on next year.

NATO combat operations will finish at the end of this year, but the Taliban have launched a series of offensives that have severely tested Afghan soldiers and police.

The new NATO mission will focus on supporting the national forces.

But fears are growing that Afghanistan could tip into a cycle of violence as the US military presence declines, with the national security forces already suffering high battlefield casualties.

In the latest stage of the NATO withdrawal, Britain on Monday ended its 13-year military presence in southern Afghanistan when the last Royal Air Force personnel departed Kandahar airfield.