US air strikes hit a Medecins Sans Frontieres (MSF, Doctors Without Borders) hospital in Kunduz in Afghanistan, killing nine medical staff, Human Rights Watch said.
The attack raised "grave concerns about whether US forces took sufficient precautions to identify and avoid striking the facility", the rights group said.
The hospital was hit several times early on Saturday during sustained bombing that was apparently aimed at insurgent forces in the vicinity.
At least nine medical staff were killed and 37 critically injured. The hospital was treating 105 patients at the time of the attack.
"The bombing of the hospital is a shocking development for Kunduz, where civilians and aid workers are already at grave risk from the fighting," said Patricia Gossman of Human Rights Watch.
"All forces are obligated to do their utmost to avoid causing civilian harm."
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Afghan police in Kunduz have asserted that Taliban fighters were firing from the hospital.
"If that were in fact the case, it would have been an unlawful use of the hospital by Taliban forces. However, given the hospital's protected status and the large numbers of civilians and medical personnel in the facility, attacking the hospital would still likely have been an unlawfully disproportionate attack, causing greater harm to civilians and civilian structures than any immediate military gain," Human Rights Watch said.
"In addition, the laws of war require that even if military forces misuse a hospital to deploy able-bodied combatants or weapons, the attacking force must issue a warning to cease this misuse, setting a reasonable time limit for it to end, and attacking only after such a warning has gone unheeded," it said.
According to MSF, the bombing continued for 30 minutes after US and Afghan military officials in Kabul and Washington had been informed of the first attack's proximity to the hospital.
"The failure of US forces to stop an attack striking a hospital strongly suggests the forces may have violated the laws of war in identifying risks to civilians and civilian objects and in the weapons used or conduct of the military operation," Human Rights Watch said.
A spokesman for US forces in Afghanistan said the incident was under investigation.