"The last 24 hours have been very violent in Afghanistan with many losing their lives. 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 wrote on his Twitter page on late Wednesday.
The diplomat has also slammed the Taliban* movement for recent attacks that also resulted in many civilian deaths.
"The Afghan people want an immediate start of peace negotiations and a settlement that is in their best interest. More graves will not bring negotiations forward. Rather than setting the process back, 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 added.
On Wednesday, a local witness said that at least 50 people had been killed in the airstrike in Herat.
On 29 February, the United States and the Taliban movement signed a peace deal in the Qatari capital of Doha, stipulating gradual withdrawal of US troops, the beginning of intra-Afghan negotiations, and prisoner exchanges. Despite the reached agreement, the situation in Afghanistan remains unstable.
*Taliban is a terrorist group banned in Russia and a number of other countries