Related Rule
Practice Relating to Rule 19. Control during the Execution of Attacks
Sweden’s IHL Manual (1991) states:
Even after a decision to attack has been made by a senior commander, the attack can be cancelled or suspended … in the following cases:
a. the objective proves not to be a military one, or to be entitled to special protection. An example of this is where military vehicles are being used as ambulances.
b. If it can be expected that the attack will cause such large unintentional civilian losses and damage that these would be excessive in relation to the anticipated and direct military advantage. In this case, the proportionality rule must thus be reapplied at a later stage. The feasibility of doing this depends to a large degree on the type of attack involved. For example, to require an assessment according to the proportionality rule from an individual aircraft pilot is probably unrealistic. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section, p. 72.