Zalmay Khalilzad, who brokered the February 29 accord with the Taliban aimed at ending America's longest war, said the Islamic State group, not the Taliban, was to blame for a horrific attack Tuesday against a maternity hospital in Kabul.

The bloodshed led Afghan President Ashraf Ghani to order a resumption of offensive operations against the Taliban, further clouding efforts to start peace talks between the two sides on a political settlement.

"The Taliban have implemented their agreement not to attack the coalition forces," Khalilzad said.

He said the Taliban, who claimed responsibility for a deadly truck bomb Thursday against an army base in eastern Afghanistan, never promised to stop violence against Afghan forces.

"They've committed not to carry out attacks in 34 major cities, and they haven't done that, based on our assessment," Khalilzad said.

"But we believe that they're in violation of the spirit, given the number of attacks and Afghan casualties in those attacks," he said, calling Thursday's bombing "very negative."

Khalilzad said that the United States was implementing its "conditions-based withdrawal" under the Leap Year deal which aims to remove all US troops by early 2021, nearly two decades after the invasion that toppled the Taliban following the September 11 attacks.

"The key requirement for the United States is the delivery on the commitment by the Taliban on counterterrorism -- number one," Khalilzad said, referring to the insurgents' promises to counter Al-Qaeda and the Islamic State group.

He said the United States was also pressing for the Taliban to start talks with Ghani's government, which the insurgents do not recognize.

"It would be best if inter-Afghan negotiations happen when we have substantial forces still in Afghanistan," Khalilzad said.

Afghan authorities have linked the Islamic State group to the Taliban, who have denied ties and condemned the maternity ward attack which killed newborns, mothers and nurses.