In this case, NATO said in its statement, its surveillance had spotted the column of military vehicles, which included tanks, in an area where Qaddafi forces “had recently been operating.” The statement added, “In a particularly complex and fluid battle scenario, it was assessed these vehicles were a threat to civilians.”

In early April, NATO admitted its warplanes twice hit rebel positions, killing more than a dozen men.

Around the same time as Thursday’s mistaken strike, rebels based in the city of Misurata were complaining that NATO had been telling their fighters to hold back from the battlefront near Zlitan to avoid getting caught in attacks on Qaddafi forces there. The rebels said NATO had failed to deliver the promised attacks and in the process slowed the rebel advance.

“If it wasn’t for NATO, we could have moved the combat line much further from Misurata,” said Mohamed, a rebel spokesman, though it is far from clear that the rebels could have held their ground without NATO support.