Afghanistan's President Ashraf Ghani has previously singled out the Taliban for the killing, but according to Khalilzad, the US government has concluded that the local Islamic State group in Khorasan, IS-K, is behind it.

Many were killed

IS "opposes a peace agreement between Afghanistan and the Taliban and tries to incite sectarian war like in Iraq and Syria," Khalilzad writes on Twitter.

At least 24 people, including mothers, medical staff and at least two infants, were murdered by armed men who attacked a hospital in Kabul on Tuesday. At least 16 people were injured. On the same day, 32 people were killed during a funeral ceremony in Nangarhar province in a suicide attack. More than 130 people were injured in the bombing.

Called war crimes

The human rights organization Human rights watch condemned the hospital attack as an "obvious war crime". The UN has demanded that the perpetrators be brought to justice.

IS has already assumed responsibility for the attack on the funeral, but no group has said it was behind the deed against the hospital.