KABUL, Afghanistan — A government airstrike killed at least 14 people in Afghanistan's western Herat province, many of them women and children, witnesses said Thursday.

Hundreds of people had gathered in Herat's Adraskan district to welcome home a former Taliban fighter freed from jail when aircraft pounded the gathering, said Noor Rahmati, a witness who lost three members of his family in the airstrike.

Government officials said Thursday the airstrike a day earlier was being investigated.

Ghulam Nabi had been released as part of a prisoner exchange aimed at moving negotiations between the government and the Taliban forward, a second and critical phase of a peace deal between the U.S. and Taliban.

District elders and well wishers had arrived to welcome Nabi when the attack began, the witnesses said. Nabi's 9-year-old son was apparently wounded in the attack.

U.S. Peace envoy Zalmay Khalilzad tweeted a condemnation of the violence and welcomed the government investigation.

The United Nations has been critical of the increasing death toll among civilians in Afghanistan, caused by both sides in the protracted conflict.

"In Herat, photos and eyewitness accounts suggest many civilians including children are among the victims of an Afghan airstrike. We condemn the attack and support an investigation," Khalilzad tweeted.

The Taliban issued a statement denouncing attacks on Taliban prisoners freed as part of the peace deal. The deal calls for the government to release 5,000 Taliban in jails throughout the country and the insurgent movement to free 1,000 government personnel, including Afghan security forces.

The statement warned that violence against freed prisoners will undermine the deal signed in February and make the job of keeping freed prisoners from returning to the battlefield that much more difficult.

"Such actions can force prisoners who are recently released and wish for a normal life to take up weapon against the enemy again," the statement said.

Witness Abdul Khaliq, whose brother Abdullah was in a hospital in Herat with wounds from the attack, said "these victims are not Taliban. They just wanted to meet a relative returning home."

"They saw women wearing their scarves covering themselves and running for safety and still they hit them too," Rahmati said. He called for an international investigation into the airstrike.

The U.S.-Taliban peace deal was touted as the best hope for peace in Afghanistan after decades of war at the time of its signing on Feb. 29. While the U.S. and NATO have already begun reducing their troop strength, the second phase of the deal — which calls for Afghans on both sides of the conflict to sit at the negotiation table — has been delayed. Much of the delays have been as a result of Kabul's reluctance to free some Taliban prisoners identified for release as part of the agreement.

For the Taliban's part, the agreement calls on them to ensure Afghanistan is not again used as a safe haven for terrorists to attack the U.S. and its allies. However, the details of how the religious movement will fulfill that commitment have not been released by Washington, citing security reasons.

The government's national reconciliation council tasked with negotiating with the Taliban issued a statement Thursday condemning the killing of civilians and added its voice to those supporting a swift inquiry.

"Unfortunately the figures from battle grounds show civilians are the main victims of the war," said the statement, which also urged both sides to protect civilians.

In January, a drone attack carried out by U.S. forces in western Herat killed at least 10 civilians, including three women and three children. The drone was apparently targeting a Taliban splinter group, according to human rights officials.

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Associated Press writer Kathy Gannon in Islamabad contributed to this report.