NATO is investigating an air strike on a column of tanks near the oil town of Brega in which Libyan rebels say five of their fighters were killed.
It is the second time in less than a week that the rebels have blamed NATO for bombing their comrades by mistake.
Thirteen died in an air strike not far from the same spot during the weekend.
The military alliance says there have been several days of fierce fighting in the area, and the situation is unclear and fluid.
NATO is filling a United Nations mandate to protect civilians in Libya through air strikes and enforcement of a no-fly zone aimed at curbing attacks on rebels by forces loyal to Libyan leader Muammar Gaddafi.
Meanwhile, a senior US general says it is unlikely Libya's rebel forces can oust the regime's leader.
General Carter Ham, who led the first stage of the air campaign, said the conflict appears to be turning into a stalemate.