Eight civilians were killed when an Afghan airstrike hit a group of people who had gathered to celebrate a Taliban commander's recent release from prison, officials said Thursday.

The strike in the western province of Herat on Wednesday drew condemnation from a top US diplomat and underscores the worsening violence in Afghanistan's war even as the Taliban and Kabul are supposed to be preparing for peace talks.

According to Ali Ahmad Faqir Yar, the district governor in the area the strike took place, a group of people had gathered to welcome the Taliban commander. "An air strike was carried out during the ceremony and civilians who participated were among those killed," he told AFP, putting the toll at eight civilian dead and 16 wounded.

On their website, the Taliban said the insurgent had been released from Bagram prison outside Kabul on Tuesday. It was not immediately clear if the commander had survived the attack or how many insurgents were killed or wounded. The defence ministry said it was investigating.

The commander had been freed under a contentious prisoner swap that is seen as a crucial step toward eventual peace talks. The exchange, outlined in a deal between the Taliban and the US in February, will see the Afghan government release 5,000 insurgents while the Taliban free 1,000 Afghan security force captives.

Kabul has protested that many among the Taliban inmates being released are dangerous fighters who will return to the battlefield. On its website, the Taliban accused the government of intensifying "raids on the homes of prisoners who have just been released".

In recent months, violence levels have soared in Afghanistan, with the Taliban carrying out near-daily attacks against security forces. In a separate incident in the eastern province of Nangarhar on Wednesday, at least 31 Taliban fighters were killed in clashes with security forces, the defence ministry said.

Zalmay Khalilzad, the US special envoy to Afghanistan, condemned the latest bloodshed. "The last 24 hours have been very violent in Afghanistan with many losing their lives," he said on Twitter.

Khalilzad said photographs and witness accounts indicated that many civilians including children had been killed in the Herat strike. "We urge all sides to contain the violence, protect civilians, and show necessary restraint as the start of intra-Afghan negotiations is so close," Khalilzad said. "More graves will not bring negotiations forward."

Afghanistan's military has a US-backed air force that in recent years has gained greater capability to conduct air strikes.